Ashburton is preparing for mass evacuations as the Ashburton River rises and heavy rain continues to fall.

At a press conference this afternoon, Ashburton Mayor Neil Brown said "half of Ashburton" will need to be evacuated if the river's stock banks break.

Three thousand homes could be flooded if the Ashburton River rises further, Acting Minister for Emergency Management Kris Faafoi says.

Faafoi is in Ashburton where he is updating the media as heavy rain continues to lash Canterbury.

Heavy rain is set to continue overnight and into Monday, so water levels are likely to rise, Faafoi said. Updates were being prepared for parents as to whether schools would be open tomorrow.

NZTA was monitoring roads and bridges.

Ashburton Mayor Neil Brown said if residents were feeling unsafe in their homes they should call 111 or leave if it's safe to do so.

"If they're feeling unsafe they should call someone, get some advice or leave. We've planned for this."

The Ashburton River is the one major concern, Brown said. Faafoi added that the next 24 hours would be critical.

Residents should stay home and "be cautious when out and about".

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Brown said while some flow monitors on Ashburton River had been damaged, others were still providing information.

In the last 24 hours, the Ashburton high country had experienced heavy rain which was making its way down through the river systems, Brown said.

There was still "quiet a bit of capacity" in the river. "We need it to stop raining to let those rivers drop," Brown said.

A welfare centre was being set up at Hakatere Marae in case it was needed.

Emergency services were prepared for the bad weather due to forecasting since Friday.

Earlier today, a state of emergency was declared in Timaru District and Ashburton.

"This is to allow council and partner agencies to carry out necessary tasks in the face of surface flooding and rising rivers due to continued heavy rainfall across the district," the Timaru District Council said in a statement.

Timaru Mayor Nigel Bowen said a state of emergency had been declared as of 11.42am.

Two Civil Defence welfare centres have been set up in Geraldine at St Andrew's Hall and the Geraldine High School library.

The state of emergency for the Ashburton District has been declared for seven days.

Earlier this afternoon around 70 residents of Selwyn Huts were asked to evacuate as rising river levels threaten to cut off access to the settlement.

Selwyn District Council is also telling Sheffield, Waddington and Springfield residents to self-evacuate if they have concerns, as surface flooding affects significant areas of the township.

Selwyn Civil Defence was going door-to-door this afternoon requesting residents to "evacuate now" as flooding of the Selwyn River was expected to cut off access to the settlement via Days Rd.

They said a flood flow breakout above the huts and over the road was "very likely to occur in this event".

More than 1000 homes are now also without power across Christchurch.

Orion has deployed crews to multiple outages - the latest being 555 homes in the Papanui and Chaneys areas.

Flood levels in Canterbury have exceeded warnings "by a huge magnitude", and multiple rescue operations have taken place throughout the day.

"It's a really serious situation here in Canterbury; the rivers are breaking in multiple places," Federated Farmers president David Clark said.

Clark has been helping with rescues, including a farmer trapped in a tractor by the floodwaters.