Huge chunks of ice have broken off the Tasman Glacier, New Zealand's largest.
They have filled up at least a quarter of the meltwater lake at the foot of the glacier in the Aoraki/Mount Cook National Park, reports say.
The lake started to form in the 1970s as the glacier rapidly retreated - a phenomenon thought to have been largely caused by global warming.
One guide says the chunks resemble huge skyscrapers lying on their side in the water.
"We've got skyscraper-size icebergs floating around on the lake," Glacier Kayaking owner Charlie Hobbs told Radio New Zealand.
Another two local guides were alerted to the event early on Wednesday morning.
The falling ice chunks led to some "chaos" on the water, Anthony Harris, a guide at Southern Alps Guiding, told the stuff New Zealand website.
A tidal surge up to two metres (6.5ft) high damaged a lake jetty and lifted a boat trailer upside down onto another trailer, Mr Harris said.
"All in all, this is the most significant event I've seen in the last five years on the Tasman."
New Zealand glaciologist Heather Purdie says large calving events - when ice chunks break off from the edge of the glacier - happen about once every two years on the glacier, and is not necessarily the result of global warming.
But the fact the glacier, located on New Zealand's South Island, has been retreating and becoming smaller over the last few years is attributable to warming, she told the stuff website.
The ice chunks breaking off are caused by glacial ice above the water melting, putting pressure on the ice underneath the water.
"The water gets in underneath the ice and sort of jacks it up, and it snaps off.
"Large calving events are less frequent, but the icebergs that come up are really big."