The Fire Service said the Port Hills' steep slopes are the main reason why the blaze has been able to spread at such alarming speed.

LISTEN ABOVE: Selwyn District Mayor Sam Broughton speaks to Mike Hosking

Principal Rural Fire Officer Richard McNamara said the extreme angle of the terrain behind Christchurch has allowed the fire to tear down slopes, fuelled by wind and dry vegetation.

"It's burning on a slope and for every 10 degrees of slope that fire is going to double in intensity and speed."

Selwyn District Mayor Sam Broughton told Mike Hosking native vegetation burns a lot more slowly than pine and macrocapa - and questions do need to be asked about what should be grown on the Port Hills.

"Those questions will be asked but in the end we're dealing with an emergency situation right now."

"Certainly once this is dealt with there will be time for that discussion."

Broughton said it's been a very unpredictable fire with the situation changing very quickly.

"There's been a fantastic effort from the firefighters, as with Police and Defence Force and many others behind the scenes.