Tourists rode rolling waves on Tasman Lake after yesterday's earthquake caused 30 million tonnes of ice to break off - "calve" - from the country's longest glacier.

At Mt Cook village the quake was felt as a minute-long gentle tremor - it did not cause any injuries. There, the 6.3 monster put on a benign and beautiful show.

Glacier Explorers had 16 tourists and two staff members out on two boats when the quake hit. At the time, they knew nothing of it.

American visitor Val Neuman said the boat he was on had gone out only 500m from the jetty at the southern end of the lake when the sound of ice splintering startled him.

"We heard a loud crack, like a high-powered rifle, looked about, didn't see much happening. We motored on for three or four minutes around the icebergs at the lower end of the lake, then one of the fellows asked, 'Well, where is the actual glacier?"'

Guide Gerry Lemon pointed to the upper end of the lake 6km away.

"At that very moment my wife focused on that area with her camera and the whole front of the glacier cut loose and fell into the lake," Mr Neuman said.

Mr Lemon said the moment was spectacular. An estimated 250m of the glacier's terminal wall is below the waterline and the slab - 1.2km wide and 75m-long - that calved its way up out of the lake was like a submarine breaking the surface, he said.

"You could see this huge big piece of blue ice coming up out of the water. It was amazing, a whole wall of water, maybe 50 or 60m high came off the ice.

"All the icebergs started rolling, every single one on the lake was rocking and rolling. They're exposing their beautiful blue undersides."

The 1.2km behemoth has since broken up. Yesterday morning there had been only 10 icebergs on the lake and now there are more than 30. Their intense colours, caused by refracted light, stand in sharp contrast to the lake's sediment-rich water.

The tourists felt rolling swells of up to 3.5m. Mr Lemon said it was safer to keep the boats pointed into the swells rather than try to offload passengers on to the jetty. In all, the group was on the lake for an hour and 15 minutes.

After nearly 2m of rain in the past eight weeks, the lake's temperature had risen, warning lake-users of a coming event.

Accordingly, the company boats had been keeping 800m away from the glacier's terminal face, Mr Lemon said, but "never in a million years" did anyone expect an earthquake - uniquely - to cause the third-largest calving in the lake's history.