A warning that the Waitaki River level in North Otago may rise quickly was issued tonight by Oamaru police.
It came in the wake of a heavy rain warning from the MetService for areas from Canterbury north to Taranaki.
Sergeant Tony Woodbridge, of Oamaru, said Meridian Energy had told police they were releasing water from all dams along the Waitaki River.
"There is a chance of the river level rising quickly so police ask anyone camping in the low-lying areas of the Waitaki Valley to be aware of the river levels and take precautions," Mr Woodbridge said.
The situation was being monitored by both Environment Canterbury and police.
Last night a sudden deluge in Greymouth badly damaged a number of houses and it will be several weeks before the residents are allowed to return.
Greymouth appeared to suffer the worst from a band of heavy rain that moved up the country last night -- prompting rain warnings from the MetService and cautions on road conditions.
Eleven people were evacuated in Greymouth, including two from council pensioner flats, when more than 10cm of rain fell in a few hours.
Some were able to return to their homes today but other houses, including the pensioner flats, were uninhabitable, Grey District Mayor Tony Kokshoorn told NZPA.
A "massive weather bomb and flash flood" saw one house knee-deep in water and the volume of water going down the road made it look like a little river, he said.
"You have to see it to believe it."
Mr Kokshoorn said it would take between two and five weeks for some of the damaged houses to be refurbished and carpets replaced.
Most of the residents were insured but a few were not, he said.
The council had met Housing New Zealand this afternoon to discuss the residents' accommodation options.
The flooding also damaged shops in the centre of Greymouth, including the IRD offices.
State Highway 6 was closed at Punakaiki, north of Greymouth, and between Haast and Makarora.
