Fire crews will work through the night in hot and windy conditions to try to contain a fire in and around Bundaleer Forest in South Australia's mid north.

The fire has burnt 2,300 hectares of mostly inaccessible terrain since it began on Wednesday night.

The blaze is travelling in a south-easterly direction, and crews are trying to prevent it from crossing RM Williams Way, which connects Jamestown and Spalding.

A wind change at about midnight is expected to push the fire away from homesteads.

More than 200 firefighters, 50 water tankers and three water bombers are on-scene and six more strike teams have been called in.

CFS state coordinator, Mark Thomason, says firefighters have spent much of the day trying to establish control lines.

"Rate of spread is a lot slower in the forest area because the wind speed's been reduced because of that vegetation," he said.

"It's only when we get into grassland at this stage where we'll see rapid fire spread.

"We've identified a number of properties in the south-eastern and eastern flank of the fire where some work is being done to re-enforce those assets.

"It's very hilly and thick vegetation, and I guess it's a concern to us that if fire gets out under those strong wind conditions then we're going to have a large fire on our hands."

'Worst ever'

Local farmer and Mayor of the Northern Areas Council, Denis Clark, has been helping firefighters since last night.

"RM Wlliams Way is between us and the fire. You never say you're safe but hopefully we can get it before it crosses that RM Williams Way. It's the worst fire I've ever seen in the Jamestown area," he said.

"It's probably only about the southern 20 per cent of the forest that's been affected at this stage but if the wind changes that will change dramatically," he said.

CFS incident controller Peter Smallacombe says stock losses are expected in grazing land to the west of the forest.

"The local farmers have moved a lot out overnight but obviously there will be some that we will find that didn't quite get there," he said.

"We have been in asset protection overnight on the southern boundary of the fire. We've been doing back burns in and around those areas."