Two people have died in a major bushfire north of Adelaide and there are unconfirmed reports of a third death, South Australian Premier Jay Weatherill says.

Fire crews are working through the night battling the blaze, which is burning out of control.

One body was found in a paddock near Pinery and a body was also found in a car near Hamley Bridge.

Key points: Two people killed in bushfire burning out of control north-east of Adelaide; fears for a third person

Two people killed in bushfire burning out of control north-east of Adelaide; fears for a third person Three people in hospital with severe burns after helping fight the blaze

Three people in hospital with severe burns after helping fight the blaze Significant property and livestock losses

The unconfirmed report is of a death near Freeling after a car was reported to have hit a tree.

"Our priority will be to access these sites safely and, of course, to notify the next of kin," Mr Weatherill said.

Mr Weatherill also confirmed at least three people who were helping fight the fire have been taken to the Royal Adelaide Hospital with severe burns.

"We also know that there are many homes, sheds, crops and livestock losses due to the active fire front, I can't give you exact figures at this point," Mr Weatherill said.

Properties in Tarlee, Freeling and Kapunda have been affected by the blaze, which had burned more than 85,700 hectares on Wednesday.

An emergency warning on Wednesday night said the main fire activity was around Kapunda, and there remained potential for structures to be impacted.

The fire front was about 40 kilometres wide and was burning between Nuriootpa and Kapunda.

Heatbreak for the residents of a house lost at Wasleys. ( ABC News: Tony Hill )

CFS chief officer Greg Nettleton said the total perimeter of the fire was 211 kilometres.

"So if you can imagine a continuous fire line from Adelaide to Port Pirie, that's the perimeter of the fire that we've got to contend with over the next few days," he said.

"Now some of that perimeter will be fairly easy to extinguish and make safe, but there will be other areas of that perimeter that will be quite difficult as it gets into more and more difficult timbered country."

The blaze started at Pinery about midday on Wednesday and travelled 50 kilometres in just four hours. The cause of the fire is still unknown.

Mr Weatherill said there were five rapid assessment teams on the ground looking at property damage and stock damage.

He confirmed there was significant stock loss at a piggery at Wasleys.

Relief centres have been set up at the Gawler Sports and Recreation Centre, the Balaklava Racecourse and Clare showgrounds.

About 1,000 people are working to fight the blaze including 70 appliances on the ground along with 340 volunteer firefighters.

Additional firefighting strike teams are expected from Victoria on Thursday morning.

SA Power Networks said outages had impacted about 22,000 customers throughout the day.

But power has since been restored to about 14,000 customers in the Barossa Valley.

About 8,000 customers will not have power restored overnight as there is no access to the power lines that run through the fire area.

SA Power Networks said it was mobilising additional resources from other areas to help local crews from first light Thursday morning.

Fire-damaged items in the yard of a Wasleys house after a bushfire passed through the area. ( ABC News: Tony Hill )

Mr Weatherill urged anyone requiring medical treatment to attend one of the relief centres or the closest medical facility.

He said there were some hospitals in the affected area, including Riverton, Kapunda and Eudunda, that are still taking patients.

"However we are monitoring the situation closely," he said.

One school was evacuated on Wednesday, and Mr Weatherill said at this stage many schools would potentially be closed on Thursday.

"For now our priority is the safety of everyone and getting the fire front under control," Mr Weatherill said.

"Depending on conditions, and if they are favourable, we know there will be at least three to four days of work ahead of us.

"Our thoughts turn to those people affected and those volunteers working through the night to protect us."

A bushfire recovery hotline has been established, the number is 1800 302 787.

Police have also set up a Register.Find.Reunite site for anyone affected by the fires.