Passengers of two explorer boats were hit with waves of up to 3.5 metres as the ice crashed into Terminal Lake under the Tasman Glacier at the mountain.
Aoraki Mount Cook Alpine Village tourism manager Denis Callesen said huge icebergs formed in the lake, which were then rocked by massive waves for 30 minutes.
The danger to passengers could have been worse if not for safety proceedures put in place when it was suspected the area was becoming unstable, he said.
"We have procedures to deal with this type of event and for some time have stayed 800 metres away from the Terminal Face as we suspected it was becoming unstable," he said.
It was the third biggest amount of ice to fall in the history of the Terminal Lake, which is now over six kilometres long and two kilomtres wide in places, said Mr Callesen.
