A 30 million tonne block of ice sheared off a New Zealand glacier just minutes after a violent earthquake devastated the city of Christchurch, officials said Wednesday.

The huge iceberg crashed into a lake shortly after the 6.3 magnitude tremor rocked the South Island on Tuesday and created waves up to three metres high for 30 minutes which rocked two sightseeing boats on the lake at the time.

The enormous iceberg -- estimated to weigh 30 to 40 million tonnes -- began ripping off the Tasman Glacier at Aoraki Mount Cook National Park accompanied by a loud noise which sounded like a rifle shot, a local tourism official said.

Aoraki Mount Cook Alpine Village tourism manager Denis Callesen said locals had been expecting a major iceberg to drop from the glacier for the past month, but the "curve ball" was that the event was caused by an earthquake.

"The earthquake that we felt here was a swaying motion for about a minute, then it stopped and then it swayed for about another minute," he told AFP.

"Within about a minute of that happening, the staff at the lake heard from five kilometres away (from the glacier) a sound that sounded like a rifle shot and then over the next two minutes all the events started to unfold.

"I have absolutely no doubt in my mind that the earthquake was the trigger."

The Department of Conservation confirmed that a "large chunk" of the glacier fell into the lake but was unable to say if this was caused by the earthquake, which was only felt lightly around Mt Cook some 150 kilometres (93 miles) from the epicentre.

"You could argue whether the earthquake precipitated it or not -- the fact is that the terminal face was about due to carve anyway," area manager Richard McNamara told AFP.

"The estimate of around 30 to 40 million tonnes of ice would be at a rough estimate about right."

McNamara said the one-kilometre long piece of ice which carved off the glacier had broken into smaller icebergs, some of which were now about 200 metres (650 metres) in length, and towering up to 50 metres above the lake.

"It carved in one big lot; a face about a kilometre long carving is a spectacular sight," McNamara said, adding that the iceberg would then have popped up to the water like a porpoise before starting to break into smaller pieces.

He said that particular face of the glacier had experienced similar events in recent years and it was not unusual to see huge icebergs in the lake. However, the 7.0 magnitude quake which hit New Zealand in September had not produced a similar event at Mt Cook, he said.