Ice chunks the size of skyscrapers break off New Zealand's Tasman Glacier

Thousands of chunks of ice, some the size of skyscrapers, have broken off New Zealand's largest glacier - a glacier that is melting and will eventually disappear.

The Tasman Glacier in Aoraki Mt Cook National Park experiences large 'ice calving' events every few years, but not usually at the scale it has recently.

The glacier has been shrinking for decades, but in the last 13 years, that's been happening at a rate of one-and-a-half rugby fields per year.

"We had a couple of thousand icebergs break from the glacier at the same time," Glacier Explorers' Pancho Saucedo told Newshub.

The Glacier Explorers tour company even caught the icebergs breaking on camera. The tour company was guiding a group when the ice began to calve.

It broke from the front of the 300-metre glacier wall under the water and then surfaced.

"Hundreds of chunks of ice kept exploding from underwater kind of like breaching whales," Saucedo said.

He said it is hard to conceive how big the chunks of ice really are.

"Some of the bigger ones out there might have a 40 or 50-storey building underwater so they're absolutely massive," Saucedo said.

Glacier Explorers' skipper Kim Mitchell said: "There's still 90 percent of it beneath the water so the sheer size of when you get to ones like this, they're bigger than the hotel, the hermitage."

Newshub went along to the icebergs and saw one estimated to be around 3000 tonnes, that's the equivalent weight of around 200 buses.

And the ice is the oldest glacial ice in New Zealand, with some bits around 500 years old.

It's melting at a rate of 150 metres a year, all adding to the retreat of the glacier with the pieces eventually melting into Tasman Lake which didn't exist a few decades ago.

"A rugby field and a half every year or 30 centimetres a day," Saucedo said

And that's why hundreds of people are flocking to our glaciers every day, with some calling it "last-chance tourism".

Glacier Explorers had been waiting 10 days to get back on the water on the day they took Newshub out.

"We have a safety line, we always stay about 500 metres from the terminal face and every year we have to go out and remeasure that so we're creeping further and further up the lake," Mitchell said.

The changing planet caught on camera - and even the watchful eye of Aoraki Mount Cook won't stop this wall of ice from continuing to shrink.