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Paddocks and roads are under water around Cust in North Canterbury, as torrential rain pummels the area.
The MetService is warning North Canterbury can expect two months' worth of rain in just two days, from last night to tomorrow morning.
Emergency operation centres have been set up around Kaikoura and North Canterbury.
Between 250mm and 300mm rain is forecast for the Kaikoura ranges and 120mm to 200mm between Banks Peninsula and Kaikoura today and tomorrow.
"A front has stalled in this area," MetService spokesman Bob McDavitt said.
"The Kaikoura and North Canterbury coasts had upwards of 50mm of rain during Sunday night and Monday morning. That's around their August average, and the rain is expected to keep going at this rate until it finally eases off late on Tuesday."
Canterbury Civil Defence's emergency coordination centre was opened this morning, with Hurunui and Waimakariri district councils having activated their emergency operations centres.
Hurunui council spokeswoman Naomi Woodham said residents in Amberley Beach had been warned about possible evacuation.
Flyers had been distributed warning residents to "err on the side of caution" and stay with family or friends if they were worried.
The council was monitoring the situation and a decision on whether to evacuate would be reassessed at 8.30pm, Ms Woodham said.
River levels were still rising and high tide was not until 11.20pm, she said.
Police and council engineers were at the scene.
Rivers were also being monitored throughout Hurunui, Waimakariri and Selwyn districts, although none have yet broken their banks.
The Canterbury Civil Defence Emergency Management Group Emergency Coordination Centre will remain activated in support of affected districts overnight.
The heavy rain has affected State Highway 1 between Christchurch and Kaikoura, causing slips and surface flooding, and motorists are being advised to drive with caution with the road down to one lane in places.
The AA Roadwatch website showed SH1 near Whales Back was closed and was expected to remain so for "a long period".
The rail line at Billy Goat Bay was closed due to a slip.
Flood controllers at Environment Canterbury were monitoring water levels and advising other agencies.
Sandbagging operations were already under way in some areas and farmers were being advised to shift stock away from low-lying water.
Sandbags were available from Environment Canterbury's office and at local council offices.
Meanwhile in the North Island, a slip in Ring Terrace, St Mary's Bay, Auckland, has seen the evacuation of a number of properties after an uprooted tree fell into the reserve adjacent to the northern motorway approach to the harbour bridge.
The fire service was evacuating the affected property and its neighbours.
Police have cordoned off the reserve below the cliff top property.
Kawakawa-Clevedon Rd, just north of Kawakawa Bay, will remain closed overnight following a slip on Sunday.
About 500 cu m of earth came down onto the road after heavy rain.
The closure means the almost 1500 people living in the area will need to continue using a longer alternative route.
Drivers should expect delays because some parts of the road south of Kawakawa Bay are only one way following slips today.
- NZPA
