The Adelaide Hills remains under threat tonight as a massive bushfire threatens dozens of communities just 26 kilometres from the city's centre.

A major emergency was earlier declared, with the fire conditions described as the worst since Ash Wednesday in 1983.

Dozens of homes are feared lost and lives remain in danger as the out-of-control fire burns on multiple fronts after an afternoon wind change.

CFS chief officer Greg Nettleton estimated the Sampson Flat fire had burnt more than 11,000 hectares and may not be brought under control until early next week.

"We've got a lot of work to do out there, first of all in controlling the fire and then cleaning up all the insecure edges," he said.

"We'll be working very hard to try and get that fire secured before hot and windy conditions that are anticipated on Wednesday."

Premier Jay Weatherill said, despite cooler conditions, the situation remained "extremely dangerous".

"There should be no sense of relief because the temperatures are somewhat lower or that there are spots of rain about," he said.

Around 300 firefighters supported by 50 appliances were battling the blaze overnight, with 100 personnel and trucks sent from New South Wales to relieve local crews.

"There has been difficulty today in some of the areas in working out exactly where the fire is due to the smoke coverage," Mr Nettleton said.

Mr Eden said 13 firefighters had so far been injured tackling the fires.

"We've been very lucky given the hundreds of people on shift around this very active fire," he said.

"Whilst it's very sad for those that have been impacted by conditions, they are at the minor level of smoke inhalation or burns to a wrist from burning embers.

"We've had no serious or significant injuries, which is a credit to the firefighters and also a credit to the community that we haven't had any fatalities associated with this outbreak."

The fire earlier reached the Kersbrook primary school, with reports flames had also spread to the Cudlee Creek primary school, but the Big Rocking Horse remains unscathed despite suggestions to the contrary on social media.

Roads remain closed around the fire ground and spot fires have been reported outside the main fire zone.

The CFS said Saturday's fire conditions were the worst it had seen since the Ash Wednesday bushfires in 1983.

Seven homes and a number of sheds have been confirmed destroyed, but the actual number remains unknown, with damage assessment teams unable to begin work.

CFS assistant chief officer Rob Sandford said the number of properties lost could number in the dozens.

Kersbrook resident David Miller said he was trying to protect his home when it went up in flames.

"There were two windows up on that side and once the second one went, I couldn't even get inside to get keys for the vehicles," he said.

"That's why I lost the vehicles and the carport. I would be silly if I said [it's] not frightening. It's scary when you hear that fire."

A large number of dogs and cats perished as the fires swept through the Tea Tree Gully Boarding Kennels and Cattery at Inglewood.

The business owners used social media to share the news that, while they managed to save in excess of 40 dogs, no cats survived when the cattery was destroyed.

The owners said they were devastated by the loss of their home, business and people's pets.

The Sampson Flat fire was burning close to homes in Cudlee Creek. ( ABC News: Nick Dole )

"We love all the animals we have in our care and are devastated by this. Our deepest sympathy to all who have suffered a loss," a post on the business' Facebook page said.

SA Water has asked residents to cut down on their water use because of supply issues on the fire front.

"Our problem is that people are still perhaps watering gardens and things like that which aren't essential at the moment," spokesman Steve Rose said.

"There are people in places like Gumeracha and Birdwood that are facing the fire front and need all of the water they can get for their roofs, gutters and everything else to protect their properties."

Meanwhile, the CFS said the threat from a forest fire at Tantanoola in the state's south-east that had burnt up to 700 hectares had reduced.

Around 2,000 firefighters have attended dozens of incidents across South Australia, with some sparked by lightning strikes as storms moved across the state.

Evacuees congregate at relief centres outside fire zone

South Australian police earlier declared the Sampson Flat fire a major emergency, giving authorities the power to forcibly evacuate people or block them from entering the region.

The bushfire is burning freely towards townships and residents across the area are fleeing to safer ground, although some have chosen to stay and defend their homes.

There are relief centres at Golden Grove Recreation Centre on The Golden Way at Golden Grove and at the Willaston Football Club in Gawler.

Police Commissioner Gary Burns said the cause of the fire had not been determined.

"An incinerator is one line of investigation we're looking at," he said.

SA Ambulance earlier said it was treating a 64-year-old man from Inglewood for suspected burns to his throat.

It is believed the man was injured while trying to defend his home.

Another resident in the area, Merv at Cudlee Creek, defended and saved his home but lost several sheds.

"All my work tools and everything have all gone," he said.

"All my wood's gone for the year, but never mind, the house is still there."

Earlier on Saturday, deputy incident controller Ian Tanner said people in the area who are unprepared should voluntarily evacuate if conditions are safe.

"There's not a fire service anywhere in the world that could put this fire out at the moment," he said.

Drop-off points are being set up to allow donations of water and high-energy food for exhausted CFS firefighters.