Four years later he was so focused on cutting carbs from his diet to suit the demands of the AFL that he only saw "bits and pieces" of the Wallabies' semi-final exit.

But as his three sporting worlds prepare to collide on a huge weekend of Australian action, Folau says it's time to deliver in his World Cup moment.

It's fitting that the only man to play at the top level in Australia's three sporting codes has his chance to shine in rugby union just hours after the AFL grand final and hours before the NRL showpiece.

The cross-code superstar has left his NRL and AFL lives behind to start forging his name as a rugby powerhouse.

But he had little interest in rugby's fortunes in 2011 when he was trying to trim down to cope with the physical demands of playing for the GWS Giants as their marquee man.

"It's a bit of a change four years later to think that I'm in this situation now," Folau said. "I was down to 95 kilograms, I was pretty light for my height. It wasn't too much hard work to get that weight back up as it would have been to lose the weight.

"I had to cut a lot of carbs – bread and rice and that sort of stuff. But to get it back on wasn't that hard."

Folau, now a powerful 103 kilograms and boasting 34 Tests for Australia, is itching for his chance to step out in front of 82,000 fans at Twickenham on Saturday night (Sunday morning AEST).

Had his career panned out differently, he could have been running out with the Brisbane Broncos in an all-Queensland NRL grand final at ANZ Stadium on Sunday.

But Folau has no regrets about the path he has chosen – which first took him to the AFL and then the Wallabies.

The Wallabies have a chance to knock England out of the World Cup by beating the hosts on their home turf this weekend.

Beating England would guarantee Australia advances to the next stage of the tournament, while a loss would mean the Wallabies need to beat Wales in the last pool game to book a quarter-final ticket.

If England lose to Australia, they will become the first host nation to be out of World Cup contention before the end of the pool stages.

The stakes will be high when the Wallabies run out, despite the match not being a final.

But Folau will draw on his years of experience in grand finals, State of Origin matches and rugby league World Cups to ensure he's unfazed.

They are the lessons he started learning as an 18-year-old who scored 21 tries, was the NRL rookie of the year and won a premiership in his first year at the Melbourne Storm.

"I have kind of tried to learn it over the years … going through some experiences didn't work for me. Trying to carry that weight or expectation of anyone else [didn't work]," Folau said.

"I've just got to do what feels comfortable for myself and obviously for us as a team, we're just excited.

"I think [in the past] I was just trying to carry too many people's expectations … You've just got to worry about yourself really. I've learnt that over the years."

Folau looms as the Wallabies' World Cup X-factor as they aim for success against England.

He made his World Cup debut against Fiji last week and made a concerted effort to get his hands on the ball more often than he has in the past.

It is a tactic Wallabies attack coach Stephen Larkham hopes causes England plenty of headaches.

"The atmosphere is certainly going to be big [against England] … there are no weak links in the opposition," Larkham said.

"[Folau] has certainly played these types of games before, there's going to be a little more hype.

"But I thought he had a fantastic game against Fiji and delivered exactly what we needed him to deliver. I think it will be the same against England – he's a big-game player."

Folau has vowed to "back my instincts" against England to make sure he has an impact and imposes himself as he did in the NRL's biggest moments.

But it could be a combination of his rugby league and Australian football experiences that prove the biggest weapon against England.

As fullback, Folau must choose when to channel his NRL days and counter-attack at England or whether to pretend he's holding a Sherrin and use a tactical kicking game to negate England's back-three weapons.

"I think it's important to make a good decision in that time … I've got to go out there and play with good brains," Folau said.

"You can't run everything, you've got to pick the right decision. You go with your gut feel really, back your instincts.

"You can't be indecisive, you can't be in-between [running and kicking], that's the most important thing."