Firefighters are continuing to battle the Port Hills blaze that has roared across 650 hectares casting a thick pall of smoke over Christchurch.

The wildfire has seen dozens of residents evacuated and a State of Local Emergency declared.

Meanwhile a second fire in the area is now threatening buildings.

Firefighters have confirmed they are responding to a grass fire that is threatening buildings in the Selwyn district this morning.

The fire was reported shortly before 9am in Rolleston.

Four trucks and one tanker have been sent to the blaze.

Back at the Port Hills blaze, 22 firefighter crews worked through the night and were being be replaced by fresh crews about 6am, when 23 appliances would be back on the ground supported by 13 helicopters and two fixed wing aircraft.

More than 100 homes are thought to have been evacuated, with approximately 50 people attending a briefing for evacuees at one of two official shelters on Wednesday night.

Residents who have been evacuated were not able to return home and cordons remain in place.

A State of Local Emergency was declared for Christchurch City (which includes Banks Peninsula) and Selwyn District on Wednesday.

"The declaration means we are better able to coordinate our response across all the various agencies involved, and will mean we can get assistance to where it's most needed. It also means the local controller can evacuate people or restrict access to affected areas as required," Christchurch mayor Phil Mauger said.

An investigation into the cause of the fire has begun.

A public health warning for smoke from the fire was issued on Wednesday by Te Whatu Ora.

Medical Officer of Health Matt Reid said exposure to smoke can worsen pre-existing health conditions including heart and lung disease.

The smoke could also cause coughing, shortness of breath or eye, nose and throat irritation for pregnant, young or elderly people.

People were advised to stay inside, restrict outdoor exercise, switch air-conditioning to recirculate, and keep pets inside.

Anyone feeling anxious could contact Healthline on 0800 611 116.

People were being urged to stay away and not fly drones anywhere near the blaze because the helicopters would need to be grounded.

Main image (Photo: Supplied/Baily Lelieveld): Fire in the Port Hills on 14 February 2024.