Visitors to Mt Cook's Tasman Glacier are warned to stick to vantage points on high ground.
Authorities say there'll be a tsunami in the lake as the iceberg has risen another 20m out of the water and will break off at some point.
The unique event's been triggered by 250 mm of rain falling in 72 hours.
Aoraki Tourism general manager Denis Callesen says signs are going up to warn people to stay away from the water's edge.
"By my calculations if nearly 30 million tonnes carve off together - one can only imagine what sort of a surge event would occur with that."
He says last time this happened a three meter surge of water was recorded.
- NEWSTALK ZB
Authorities warn of Tasman Glacier tsunami
Looking over the icebergs, down the Tasman Glacier to the valley below. Photo / Sarah Ivey
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