A blaze that's ripped through 300 hectares of conservation land in Canterbury and damaged a major rail line has continued to burn on numerous fronts overnight, with fire crews now on the ground.

But the Department of Conservation says it's optimistic about getting the fire - located in the remote Waimakariri Gorge - under control on Sunday.

A crew of about 50 staff and 10 firefighting helicopters are now trying to contain the fire that started on Saturday morning and continues to billow smoke across the Canterbury Plains.

Department of Conservation North Canterbury operations manager Kingsley Timpson said ground crews had been at the scene overnight keeping a watch.

He said while it was still burning on "numerous different fronts", winds were expected to be calmer on Sunday before picking up again next week, creating a chance to get the blaze under control.

The fire, between an area known as Staircase and Craigieburn, has torn through 300ha of scrub and alpine forests on public conservation land.

It's closed KiwiRail's Midland trail line, which runs through the area, affecting TranzAlpine passenger train and freight services.

KiwiRail's group general manager, network services, Todd Moyle on Saturday said "we do know already that there has been some damage to bridges and signals equipment".

"A full assessment of damage to the track will be carried out when the fire is under control and it is safe for our staff to access the site.

"Only when that has been done will be able to comment on when the track will be open again," he said.

Ground crews were trying to protect rail bridges but an assessment of the Broken River viaduct was yet to be undertaken, Mr Timpson said.

The viaduct is one of the four major viaducts on the Midland Line from Chirstchurch to Greymouth.

No one has been injured and the cause of the fire is being investigated.