Christchurch's Mayor is vowing the city won't be caught out by ex-cyclone Gita.

Lianne Dalziel told Andrew Dickens heavy rain is due to set in any time between 5pm and last until 1am.

She says the city's been caught out by an unpredictable storm event before.

"And I'd much rather send the message that this is serious, we're taking it seriously. It doesn't mean that it's going to be as bad as it was in July, but it could, and what's worse is that it could be worse."

She is urging people to get off the roads and to a safe place before darkness falls, which was one reason for declaring the state of emergency now.

That particularly goes for people living along the Heathcote River.

"If you've flooded before or it got close to flooding before, you really ought to think about heading out and staying with family and friends. We will be setting up a welfare centre at Pioneer Stadium just in case."

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LISTEN TO LIANNE DALZIEL TALK WITH ANDREW DICKENS ABOVE