As the blinding wind whipped at his face, he turned on his headlamp and looked at his watch - it was 10.30pm. Soon, he would realise a dream that had begun gnawing at him 27 years earlier in the back of a pub in Wagga - a quest to become the only Australian to climb every mountain in the world higher than 8000 metres, all without supplementary oxygen. He had returned to where it all began - Everest - saving the biggest of them all for last.

As he stumbled slowly towards the summit, Lock began playing a mind-game with his body, testing it to see how far he had pushed himself into the red, and how much further he had left to go. The vomiting and headaches had arrived long ago, as expected, as the cerebral oedema became more and more intense.

With his climbing rope flailing out horizontally from the roaring blizzard and struggling to put one foot in front of the other, he paused to reassess. It was not the first time he had been denied his prize and he was not about to give it up lightly.

A year earlier he had nearly been torn off the side of Everest while still inside his tent when a storm moved in, and on a previous attempt his climbing companion had become overwhelmed by altitude sickness near the summit, forcing him to descend to save his friend's life.

Standing alone in the darkness as conditions worsened, he pushed on.

"Interestingly, the oedema hadn't yet affected my ability to make rational decisions about my physical condition, so I figured it was safe to keep going."

Cerebral oedema, an often fatal swelling of the brain at altitude, has been known to not only cripple climbers physically, but also to send them mad: tales of climbers stripping off their down suits in the middle of blizzards, abusing sherpas for trying to save them from certain death and hallucinating about sitting in a comfortable sofa while dangling from the end of a rope are common.

At 8500m, with just 300m of climbing left to go, he looked at his watch again - 1.30am. As he advanced towards the final push for the summit, a black tunnel came rushing towards Lock as his vision finally began to shut down.

"It's hard to describe, it was kind of like giant black bats rushing towards me, and it was at that point that I knew I had to turn around."

Returning to his home in Canberra after that failed attempt in 2012, Lock, one of the most successful Australian climbers in the country's history, announced he was retiring from 8000m peaks. For half his life he had played a deadly game on the world's most treacherous mountains and won, while watching dozens of friends and climbing companions die around him.

He had had enough of the pain, the grinding cold, the broken relationships and the anxiety of wondering if this climb was to be his last.

What did he really have left to prove, anyway? In the annals of climbing greats he was already among giants, the first and only Australian to successfully climb all 14 of the 8000m peaks, the first Australian to summit not one, but six of the Himalaya's highest mountains, and he had already climbed Everest twice using oxygen. Why take those risks again, just to do it without the help of a gas cylinder?

It was over, and he would have to be satisfied with having climbed 13 of the 14 highest mountains - the so-called grand slam of mountaineering - unassisted.

But just over two years later, sitting in his Campbell living room, Andrew Lock is not so sure any more. Is he really done? Can he really leave this one, final project incomplete, or does he have one more climb left in him?

A sly smile spreads across his face when I ask if he's had a change of heart.

"I've certainly thought about it," he admits.

"Climbing Everest at 53 without gas." He lets the words hang in the air as if studying the idea that has just come out of his mouth. "That'd be a tough one, and a record for me."

"For me, it was always about Everest, the dream that began with a slideshow in Wagga in 1985 [a presentation by Tim Macartney-Snape and Greg Mortimer, who a year earlier had made the first Australian ascent].

"It was the initiator, it took me 15 years from setting the goal to achieving it, and I guess it's ironic that it was the beginning of the dream, and it's still the end of the dream too.

"I haven't done Everest without gas, and I would still like to do it without gas."

Andrew Lock is the most successful athlete most Australians have never heard of. While lesser climbers were chasing publications and seeking sponsorship deals for feats such as climbing the highest summits of each of the seven continents (which counts the gentle stroll to the top of Mt Kosciuszko as one of its summits), Lock has spent a lifetime pursuing a harder path, seeking out the remote, less-climbed routes, often climbing alone, faster, for longer and mostly without the assistance of high-altitude sherpas.

While overseas friends have gone on to become professional, sponsored climbers, he has cobbled together jobs as a police officer, tour guide, and Canberra public servant, each job lasting just long enough to finance the next expedition, or until his boss at the time eventually tired of the extensive leave requests and long absences from the office.

"There are media tarts out there, and they're very good at getting attention for themselves, they love the media and the media loves them. Andrew's not one of them," says Canberra Climbers' Association president Zac Zaharias, who has climbed with Lock on both Mt Cho Oyu and Dhaulagiri.

"Andrew has paid a price for that, and he would be a household name if he was European. Australians just don't really get mountaineering, they don't understand just how significant his achievements have been."

At 53, Lock would be one of the oldest mountaineers to climb Everest without additional oxygen. He also refuses to take on the crowds on the popular northern route during peak season, preferring the more difficult post-monsoon season.

Zaharias believes it may still be possible, but there are potentially greater challenges ahead for his friend.

"He might do it, I think deep down he'd still like to, but as you get older it gets harder. Maybe the ambition is still there, but the reality is that it might not happen.

"He's given it two big goes (in 2011 and 2012), and I feel the bigger issue for me is what do you do to take up that level of intensity or challenge when you've decided you're not going to do that anymore? I think that's the hard bit. I know he's into other challenges but they pale into insignificance compared to what he's done.

"When you're up high it's just an extraordinary place to be, it's special. Mountaineering provides a certain clarity, you just shut all the other bullshit out that goes on in life, and that clarity is hard to give up or replace."

One of the hallmarks of Lock's climbing career has been an almost cat-like ability to avoid death.

"He has this almost sixth sense, he knows when things aren't right he turns around and goes down when others push on, I believe that's a big factor in why he is still alive," says Zaharias.

"People think it's something metaphysical, I think it comes from a lot of experience, it's being more rational, knowing that the factors aren't right, the snow's not right, and turning around when others might go on."

It was that sixth sense that saved Lock from almost certain death in 2004, when a feeling of dread came over him as he approached the final camp on Everest before the summit, despite being less than a day away from an oxygenless ascent.

He turned around and descended while the rest of his party pushed on to the fourth camp. That night a storm that had not been forecast blew in, trapping the other climbers in their tents for several days while the blizzard continued, all the while using up their oxygen supplies.

"Had I, with no oxygen, gone to camp four, at 8000m, and then become trapped in that blizzard, I probably would not have survived," he later wrote in his mountain journal.

In June 2014, the phone rang again in Lock's Canberra home. It was an old climbing companion - was he interested in joining their expedition to Everest? He declined, because the timing and plans for the expedition were not the right fit.

One of those who believes he will return to complete his mission is Welsh climber Neil Ward, who has climbed three 8000m summits with Lock.

"I myself was hoping to complete Everest with Andrew, to pull him out of retirement.

"I do believe his quest for adventure has not been quenched. So we shall see if he will do something else just as challenging, even if it is not a mountain. Like myself; it's not just the mountain, it is the challenge. Life just sitting at home can get very dull."

When climbing together on Shishapangma in 2007, Wade fell through a hidden crevasse and found himself dangling upside down from a safety rope attached to his companion. Over several hours Lock and the sherpa they were climbing with slowly hoisted him to safety.

"Andrew saved my life on Shishapangma, which I will never forget. But there is also an inner switch in people. Burn out? I don't think so. Finance can be a big driving force, however a warning shot over the bow about health and safety is probably nearer the mark. How long can you climb big mountains and get away with it? Even with all the experience and knowledge as a climber you are never really in control of the mountain."

While claiming not to be one for sentimental reflection, it is clear from his journal that Lock has thought long and deeply about what he does, why, and what it has cost him.

After surviving a heart-stopping traverse across a brittle crust of ice that threatened to let go at any moment on the face of Mt Annapurna in 2007, a mountain that at the time had killed half as many climbers as those who had reached its summit, he wrote:

"You tell yourself that this is risk management, that the conditions are OK, that this slab will hold. Rubbish. You have no idea really. It is blind luck, a game of chance where the winner's reward for staking his life is a temporarily sated ego. The loser dies, as so many have. What drives this madness?"

I return to Lock's Campbell home to find the front door wide open. Getting no response from the door I follow the distant buzz of a lawn mower around the back to find him battling the overgrown garden.

"It's turned into a jungle again," he says, clearly not relishing the mundane domestic nature of the task.

A simple Nepalese piece of art on the entryway sideboard is one of only a few hints of his remarkable career. There are no oversize hero shots from the summit of Everest on the living room walls or magazine articles on display. The house has a feel of somewhere temporary, a base to operate out of perhaps, but not the sort of home that has become a life's work or a cherished project that has had countless weekend trips to Bunnings invested into it.

In June Lock will return to Pakistan to attempt another Himalayan route - "This one's just over 8000m, and yes, I know I said I was retired from 8000m peaks" - that has proven so difficult that despite several attempts by some of the world's best climbers it has never been successfully climbed.

Then the corporate work, the speaking circuit that has become his main occupation, and a number of smaller trips will keep him busy.

He worries that Everest's popularity and the unrestricted number of climbers is turning it into a circus, as large commercial groups drag hundreds of unskilled, paying clients up to limited camp sites, thereby squeezing out the private climbers.

"I trekked into base camp last year on the day of the avalanche [which killed 16 guides, one of the worst tragedies in the mountain's history] and stayed to watch the political fallout. It was like Heathrow, helicopters coming and going, streams of yaks and people at base camp. It's a little city, but it's unregulated and it's a mess.

"When I first went to Everest in 1991 there were probably 30 people on the mountain, now you've got 300 to 400 commercial clients and maybe another 500 to 600 sherpas supporting them; it's 1000 people on the mountain on one route, its crazy.

"I think a polar thing would be a good psychological challenge, it does lack the risk to life though," he adds a little disappointedly, saying it would have to be something significant that no one has ever done before, "not just dragging a sledge to the north pole wearing an orange beanie instead of a white one".

"I'm in conversations with a couple of polar guys and there might be a worthwhile adventure there. I'm not giving too much away."

He's also considering extended ocean yacht touring to remote areas as a possible fix for his need for adventure.

"For me it's the extended challenges, whether it's on a mountain, or sailing or polar. Everyday life is not very challenging, too long in normality wears me down, I need those extended challenges to charge me up."

But will sailing or trekking in the polar regions ever be able to fill the void left by Everest? Will he be able to say no to the next phone call that comes, inviting him to complete his life's work with an oxygen-free summit, one last climb?

The sly smile flashes again. "Never say never."