Up to 50 million tonnes of ice has fallen off New Zealand's largest glacier.

The Tasman glacier has changed from a U shape to an L after shedding the ice.

Mount Cook Alpine Village general manager Denis Calleson says a trail of huge icebergs has been left behind.

One is believed to be the largest in a fresh water lake outside Antarctica.

The event was thought to have triggered a three-metre-high tsunami in a remote part of Mount Cook National Park.