Swimming with icebergs is not something everyone aspires to do, but according to outdoor swimming enthusiast Peter Hancock, it should be.

The adventurer edges his way along a rocky glacial moraine wall bound for the terminal lake of New Zealand's Tasman Glacier.

It is a starry 5:00am, the dark kind where city lights fail to stain the Milky Way, and Mr Hancock is clearly excited about the swim.

"I've never swum with icebergs before," says the father of four, who is based in the New England region of northern New South Wales.

While that might not sound surprising, the icebergs have been calling Mr Hancock for a while.

He has been pursuing outdoor swimming for more than 20 years and has not missed a daily swim for almost 2,000 days.

After an hour of walking and scrambling, Mr Hancock arrives at the lake's edge and picks an iceberg.

Mr Hancock swims past an iceberg in Tasman Lake. ( ABC New England: Matt Bedford )

He pulls a thermometer from his bag to test the water temperature.

"The lie detector," he says.

"It's the one thing I never leave for a swim without."

The thermometer reads 2.4 degrees Celsius.

"Not a record," he says.

That title went to a swim in the Fox Glacier river on New Zealand's South Island eight years earlier at 1.7C.

'The shock is what gets people'

Unfazed, Mr Hancock strips off and enters the water wearing only his cap and goggles — togs are optional in the world of outdoor swimming.

In a blink, he is submerged and into his stroke, beelining for the iceberg.

"The initial shock of water this cold is what gets most people," Mr Hancock says.

"You've got to stay in control of your breathing."

Outdoor swimmer Peter Hancock swims through an iceberg in Tasman Lake. ( ABC New England: Matt Bedford )

For the inexperienced, sudden immersion in cold water could trigger an automatic response to inhale, which can be potentially deadly.

After a few minutes, Mr Hancock has lapped the iceberg several times.

His skin is glowing purple and red but he looks relaxed and elated.

Cold-water swimmers often report an endorphin rush.

"It gives you a bit of a buzz," Mr Hancock says.

The dangers of the 'afterdrop'

There is an associated danger with swimming in extreme cold and it is clear Mr Hancock is keeping an eye on the time.

He exits the water and moves straight into a rehearsed routine to warm himself.

Once dressed, he explains the danger of 'afterdrop'.

"I come out cold, but then as the blood from my extremities returns to my core, I'll start to get cold again," Mr Hancock says.

"That's probably the most dangerous part — after you get out of the water.

"The rewarming, and preparing for that, is very important."

Not long after Mr Hancock is dressed, the shivers began.

"Here they come," he says.

Peter Hancock warms himself after swimming with icebergs. ( ABC New England: Matt Bedford )

Dr Garry Couanis, a specialist sports and exercise physician and former long-distance open-water swimmer, said that while you could train your body for cold water endurance, there were many risks involved.

"Cold water cools the body faster than almost any other medium," he said.

"The reason there is so much risk of hypothermia is as warm-blooded animals we rely on … stored energy in our muscles to burn it to generate our own heat.

"The dangers of hypothermia is that as the body shuts down, the brain gets confused, the heart rate slows, and a slow heart rate puts you at risk of developing arrhythmias.

"It's a very, very risky endeavour."

So, why?

It is clear Mr Hancock loves swimming in the same way most people love chocolate, or the way children love bubbles.

It gets him up in the morning and makes him happy, but why swim with icebergs and be left shivering?

Why swim in his lunchbreaks on busy work days when time is short, or in the dark, or in the snow?

"It's an adventure."

He says outdoor swimming can be a way to find solitude, or community, or whatever you want.

Mr Hancock finds another spot nearby and records the water temperature at 1.4C for a new personal record.

"Icebergs, glaciers and a new record," he says.

"It's been a trip of firsts — that's a pretty good 'why'."