Farmers are on high alert and children have been sent home from a local school as the Tasman District braces for flooding.
The Aorere Valley in Golden Bay is expected to bear the brunt of the bad weather as a slow-moving front brings heavy rain and gales to the upper South Island.
Nelson Tasman Civil Defence manager Roger Ball said the Aorere River, which flooded in December last year after rising to a record 8m, had today swollen rapidly to about 5m.
Last year's floods damaged houses and cut off one farm in the Aorere Valley for weeks.
"It did millions of dollars of damage on that occasion -- probably the worst natural disaster we've had for many decades," Mr Ball told NZPA.
Today's heavy rain was expected to erode temporary flood control work carried out since last year, breaching riverbanks and flooding farmland and roads sometime this afternoon.
"To be honest we expect that -- that whatever you put in is going to be flooded, because you can't stop the weather," Mr Ball said.
The Aorere River level was not yet excessive, but Civil Defence was watching the trends.
"It's come up quickly and it will carry on this afternoon," he said.
The rivers in the region tended to be fairly short but if weather lingered, localised flooding could happen.
Collingwood School this morning sent children home as a precaution before roads became blocked.
Farmers have been advised to move stock, and residents and recreational users have been warned to keep an eye on fast-rising rivers and streams.
The MetService this morning kept in place a severe weather warning for the upper South Island and lower North Island.
Severe weather forecaster Heath Gullery said the front was continuing to move up the country as expected, with heavy rain and strong northerly gales coming ahead of it.
"It's basically stalled across the upper South Island throughout today, and then the front makes slow progress across the North Island tomorrow," he told NZPA.
The ranges of western Nelson and northern Marlborough were expected to receive about 180mm of rain, with up to 150mm expected in the Buller ranges.
"That heavy rain is expected to continue right throughout today and then clear tomorrow morning," Mr Gullery said.
Heavy rainfall warnings remained in place for the Tararua Ranges and Mt Taranaki from tonight, while Tongariro National Park and the eastern Bay of Plenty ranges would receive warning levels of rain from tomorrow through Friday.
Rivers and streams were likely to rise quickly and surface flooding and slips were possible in badly-hit areas.
Mr Gullery said a comprehensive weather watch covered most other North Island areas for either heavy rain or strong winds.
