Authorities are prepared for the swollen Ashburton river to breach its banks.

The region's one of three - along with Timaru and Selwyn - to declare a state of local emergency after hours of heavy rainfall.

District Mayor Neil Brown says he's never seen the river this high.

But he says the region has planned for a one-in-200-year weather event, and stopbanks are holding the river back.

Acting Minister for Emergency Management Kris Faafoi told The Weekend Collective up to 4,000 people are at risk of evacuation in Ashburton if the river breaks its banks.

"Be prepared to leave if you get the message to leave through friends or neighbours or through the likes of a emergency cellphone alert."

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