A state of emergency has been declared in Christchurch City and Selwyn District, the areas' mayors have announced, but Civil Defence Minister Gerry Brownlee says it's come too late.

The declaration follows the evacuation of 200-300 residents on the outskirts of the city.

Both Christchurch Mayor Lianne Dalziel and Selwyn District Mayor Sam Broughton said the city needs to band together to ensure the safety and welfare of residents.

"Christchurch needs a multi-agency response given the seriousness of the situation. We need to be able to draw on all the resources possible to give our residents confidence in the ongoing response," Ms Dalziel says.

But Mr Brownlee told Newshub he was concerned Civil Defence was not "crisp enough" after Kaikoura and this was another prime example.

He said Defence Force logistics officers had been in Christchurch today to see what was needed and larger numbers had been activated.

Some people in the suburbs of Halswell, Westmorland and Selwyn have been evacuated from their homes.

Houses on Kennedys Bush Road and its intersecting streets are being evacuated from Halswell Quarry up towards the end of Kennedys Bush Road. Those who have left their homes should head to Halswell Library.

