A 200-metre long ice break-off from Tasman Glacier into the lake of meltwater at its foot was expected to be massive -- but the weekend's shattering turned out to be two or three times bigger than expected.
Aoraki Mt Cook Alpine Village Ltd tourism general manager Denis Callesen -- who had predicted a "massive" calving of more than 10 million tonnes of ice, said it was actually up to five times bigger.
"The calving that's occurred is way bigger than we ever imagined," Mr Callesen told NZPA. "It's more like 30 to 50 million tonnes."
Scientists have predicted climate change will see most of the glacier melt over the next 20 years.
The glacier is the largest in the Southern Alps and, at 29km, is noted as one of the longest in the world's temperate zones, but it is at such a low altitude -- 730 metres above sea level -- that it melts rapidly.
The latest calving has shortened it by 200m.
In 1973 there was no lake in front of the glacier face, but its formation -- initially as a small sinkhole in 1975 -- was a "tipping point" according to a glaciologist, Trevor Chinn.
"No amount of snow at the head of the glacier, the neve, can compensate melting triggered by the lake."
The lake was expected to eventually more than double in length, to a maximum of about 16km.
The monster iceberg shattered after breaking off, and there are at least 20 "significant" bergs floating in the terminal lake, all at least 50m high, said Mr Callasen.
There were also thousands of smaller pieces of ice, covering about a third of the lake, he said.
His tourism company providing boat trips on the lake will likely stay away from the western side, because there was a lot more ice underwater waiting to break off and surge to the surface.
But the eastern side looked safer and it was possible that tourist boats might be able to safely get much closer this summer, after the risk of calving meant they stayed back 800m last season.
"We might be able to get within 100m," he told NZPA. The tourist boats won't go into the lake until next week.
The lake was getting deeper as the ice retreated, and was now between 180m to 200m deep.
"The scale of this enormous -- the chunks of glacier breaking off are 1000 feet (305m) from top to bottom".
