Mayors Lianne Dalziel and Sam Broughton made the joint decision in relation to the Port Hills fire, which began on Monday night and has resulted in the death of a helicopter pilot who was helping to fight the flames.
"It is no longer just a significant rural fire on the boundary of the city. It is now a city issue with suburbs being evacuated', the mayors said on Wednesday.
There are reports of 1000 people evacuating, and up to 400 forced to leave by authorities on Wednesday night, after a worrying shift in wind direction to the north-east, which significantly changed and helped spread the blaze.
Late on Wednesday night evacuations were happening these areas:
• Worsley Rd
• Hoon Hay Rd - from Cashmere Rd, and
• Kennedys Bush Rd - from Cashmere Rd
• Dyers Pass Rd has also been evacuated from Sign of the Takahe
• Early Valley Rd
Other residents should be prepared to leave their homes at short notice.
Evacuation centres have been set up at Te Hapua Halswell Centre and Nga Hau e Wha Marae.
About 120 firefighters, 15 helicopters and two aircraft were batting the blazes at Governors Bay and Marley Hill late on Wednesday.
The fires were within an estimated 1655ha area, the Christchurch City Council said, but not all of that area was alight.
The Marley Hill fire has spread extensively down Worsley Spur, causing extensive vegetation burning in the Christchurch Adventure Park. The fire was approaching the residential area of Westmoreland on Wednesday night. The Early Valley Rd fire area was also growing.
Three homes were destroyed on Hoon Hay Valley Rd on Wednesday and one on Old Tai Tapu Rd. Another house was lost on Tuesday.
The council said anyone concerned about their safety and wanting to leave should do so.
A health warning has been issued for anyone suffering respiratory issues to contact their GP.
Christchurch Mayor Lianne Dalziel said a city response was also required 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."
Selwyn District Mayor Sam Broughton said they acknowledged the "huge response effort" to so far.
"However this declaration recognises the seriousness of the situation, and this will allow us to provide all the assistance necessary to respond to the unfolding situation. The district and the city must work together to manage the situation and address the different challenges the fire is creating in each area."
Winds over the past three days have fanned the wild bushfires from the north-west and south-west, but early on Wednesday afternoon shifted to north-east winds. This pushed the blaze into new unburnt vegetation and was "not contained", Selwyn District Principal Rural Fire Officer Douglas Marshall said.
Stocks of fire retardant were running low in New Zealand and more would be needed soon, he said.
The Christchurch Civil Defence Emergency Operations Centre has been reactivated to help with further evacuations as the fires moved closer to properties. People should stay away from the hills to allow emergency services easy access.
Nearly 84,000 homes lost power in Christchurch on Wednesday afternoon, but Orion had since restored it.
Defence Force called in
Civil Defence and Defence Minister Gerry Brownlee has asked the Defence Force to provide support to local authorities in Christchurch as they deal with the fires.
Logistics personnel and equipment will be sent from Burnham Military Camp, on the outskirts of Christchurch, along with engineering equipment, water tankers and manpower options to support cordons and firebreaks, Mr Brownlee said.
Defence Force firefighters will also be used, with six firefighters and two appliances already helping at the Early Valley Rd fire since Tuesday, he said.
Plea not to use drones
Police are asking people to not use drones in the Port Hills area where firefighters are working. They are concerned that people have been using them to photograph the blazes from above.
"While we understand the appeal here, it is important people stop doing this, as it poses a major safety risk to helicopters," a police spokesman said.
"There are a number of aircraft flying in the area as they try to put out the blaze and for that reason the Civil Aviation Authority has enforced a restricted airspace."
