The glacier retreats by 180 metres a year.

The glacier retreats by 180 metres a year. Photo: Alamy

Elissa Blake admires icebergs of all shapes and sizes in a spectacular alpine lake left by the retreating Tasman Glacier.

The cold isn't the first thing that strikes you. It's the quiet. Apart from the slightly laboured breathing of my fellow glacier tourists after a hustling 30-minute alpine hike to the shores of this meltwater lake at the tongue of the Tasman Glacier, you hear almost nothing.

We're deep within the Aoraki/Mount Cook National Park in the South Island of New Zealand, and kilometres from the nearest sealed road, which runs up to the Hermitage Hotel and motel complex from where this tour begins.

But there doesn't seem to be anything else making a sound, either. No birds, no trees. In fact, apart from this newly arrived gaggle of sightseers, the rough, rocky shores of the lake appear devoid of life, save for a few grasses and mosses.

This, says our guide, Martin, as he kits us out in lifejackets stored in a tin shed and shepherds us aboard a pair of bright-yellow, plastic-hulled Mac Boats, is because there is little in the way of fauna and flora here yet. This is newly exposed land, less than 40 years old. Nature hasn't had time to move in.

In the early 1960s, the Tasman Glacier had no terminal lake. It was just a series of ponds. Since then, rising temperatures have resulted in the glacier - New Zealand's largest - melting from its tip. Since the 1990s, it has been retreating at an average rate of about 180 metres a year. The lake, which freezes over from late May to late August, is now several kilometres long, about 250 metres deep and occupies the zone between the Tasman Glacier's snout and the huge wall of heaped rock it bulldozed ahead of itself at its fullest extent.

In September, the lake thaws enough to allow small tours. Curious nature lovers can touch 300-year-old icebergs floating in the lake and observe the retreating glacier. It is one of only three tours of its kind in the world.

Settled in, eight or nine to a boat, we zoom away from the shore. Lumps of ice floating in the water clatter against the hull.

The lake is biologically dead, Martin shouts over the outboard motors. Not because it is cold, he says (and he insists we dip in an ungloved hand to get an understanding of just how very cold this water is - about 2 degrees), but because of the powdered rock suspended in the water, which turns it milky. Sunlight can't penetrate, which in turn prevents the growth of photosynthesising micro-organisms, which are the first link in any food chain. So no plankton, no bugs, no fish. Nothing.

"You can drink the water," Martin says - some of us take a sip, it's chalky, though not unpleasant - "but you wouldn't want to drink it every day. There are a lot of minerals in it." You would fur up like a kettle.

As we skirt around icebergs, we dip our hands in the water to fish out fantastic curlicues of clear ice sculpted by the action of the water. The bigger bergs come in all sizes and shapes. Some are the size of a refrigerator, others the size of a car. One or two are as a big as suburban houses. Martin approaches them cautiously. The adage about what lies beneath applies and, besides, "it's a dynamic environment", he says.

"Dynamic" as in you don't know what will happen or when. Huge rocks can come crashing down from the recently exposed cliff faces above the lake. Icebergs can suddenly flip over, shifted from delicate equilibrium by the bow wave of an approaching boat.

Martin sidles the boat up to the biggest of them. We reach out to touch it. There's a surprising range of colour and textures: the hard "blue" ice has only just been revealed as bergs turn over; the crackled white ice is being shattered internally by expanding gasses as the ice is warmed by the sun; fragments of rock catch in the light. My seven-year-old son swears he can see an insect frozen into the ice. Quite possible, Martin says. In which case the bug might be several hundred years old.

We're given plenty of time to marvel and touch and photograph before Martin guns the engines and turns to approach the tip of the glacier itself.

About 200 metres from the wall of dirty ice, which rears up about 50 metres from the surface of the lake, he kills the motor and we cruise to a stop. "This is about as close as we can safely get," Martin says. "Any closer is a no-go zone." Dynamic environment, remember?

One-thousand-tonne chunks of ice regularly fall from the glacier's tip. Any boat caught under one would be crushed, or swamped by the resulting wave. One tour party got the thrill - or scare - of a lifetime in 2011 when the Canterbury earthquake dislodged an estimated 30 million tonnes of ice and generated 3.5-metre tsunamis on the lake. Happily, the guides are trained for just such events. No one ended up in the drink.

Nothing like that today, though. The lake is like a mirror, reflecting the ice-capped mountains. Then we hear a loud ripping sound from the glacier, a reminder that nothing here is as still as it first appears. That goes for the weather, too. Up here, rain and snow sweep in quickly and a tour that began under blue skies 90 minutes ago finishes under heavy cloud.

Ashore again, we stow our lifejackets and trek back over the wall of the lake and along the winding track to the picnic area where the bus waits to take us back to the Hermitage and a huge log fire - with free marshmallows in a jar, ready for toasting.

It's impossible not to be awed by the rough beauty of the lake. It's a landscape unlike any other I've seen.

It's also impossible not to come away with a sharpened awareness of what increasing temperatures mean for alpine areas such as this and just how quickly a small-seeming change in mean temperatures can affect an ancient landscape we like to imagine as permanent.

FAST FACTS

Getting there Air New Zealand has daily services to Queenstown via Christchurch from Sydney and Melbourne, from $650 return. On certain days of the week, there are non-stop flights to Queenstown from both cities, although prices may vary. From Queenstown, the drive to Aoraki/Mount Cook takes about 3½ hours, depending on weather and road conditions in the high country. From Christchurch it is about seven hours. Newmans Coach Lines run the daily Mount Cook Wanderer, a Christchurch-Mount Cook-Queenstown (or reverse) service.

Staying there The Hermitage Hotel is the immediate area's sole hotel/motel option, offering a range of accommodation, dining and activity packages ranging from $NZ199 ($156) to $NZ429 a night, twin share. Bookings are essential (hermitage.co.nz). The nearest towns are Twizel (67 kilometres) and Lake Tekapo (104 kilometres), both offering a range of accommodation options.

When to go Glacier Explorer Tours run from early September to May 31 (depending on weather conditions). Adults $NZ145, children $NZ70.

For more information See glacierexplorers.co.nz.