An emergency warning for a fire in the Adelaide Hills has been downgraded, but the blaze is continuing to burn uncontrolled.

More than 2500 hectares of land have already been burnt in the Cherry Gardens fire and the Country Fire Service (CFS) says it expects several homes have been lost.

The fire is burning uncontrolled in a north-easterly direction but the CFS says changing wind conditions could cause the fire to change direction unexpectedly.

A watch and act message applies to a broad area, and residents in the towns of Heathfield, Longwood, Mylor, Bradbury, Biggs Flat, Scott Creek, Cherry Gardens, Mount Bold and Chapel Hill are being advised to leave if it is safe to do so.

“We still have significant resources on this fire,” CFS state duty commander Sam Quigley said.

“Additional aviation assets have been re-engaged since the sun’s up and they’re now on standby and now active in the air on this fire, with a large number of ground resources.”

Damage assessment crews are trying to work out how many structures have been lost in the blaze.

A cooler change and possible rain is expected later on Monday.

Homes lost in fire

About 300 firefighters and 50 fire trucks are on the fireground in steep terrain.

CFS deputy chief officer Andrew Stark said on Monday morning “a number of homes” had been lost in the fire.

Some onlookers likened the Cherry Gardens bushfire plume to a volcano. Photo: ABC NEWS: Samra Teague

He said several buildings had been damaged by the fire around the Mount Bold Reservoir and the Scottsdale area.

“We don’t know how many of those buildings were just farm sheds and how many were homes, but our rapid assessment teams will be going in, so we’ll have that information later on today,” he said.

He said no serious injuries had been reported.

“Everyone’s pretty safe and well, but very hot conditions for our firefighters, so we’ve had some problems with heat stress and dehydration, but nobody has been required to be taken to hospital,” he said.

Samra Teague captured this photo of the Cherry Gardens bushfire from near Willunga. PHOTO: ABC NEWS: Samra Teague

He said the fire would take a few days to extinguish, and people could expect to see smoke from the Adelaide Plains during that time.

The South Eastern Freeway is open to all traffic but there are road closures across a large area around the fire.

Smoke is blanketing the Adelaide metropolitan area, which the CFS said was all from the Cherry Gardens fire.

SA Health said the smoke could pose a serious threat to health, particularly for vulnerable people with lung or heart conditions.

“We are urging people to stay indoors where possible, avoid physical exercise outdoors and reduce their exposure to smoky air,” Deputy Chief Public Health Officer Emily Kirkpatrick said.

“People should keep windows and doors shut, and ventilate the house once the smoke clears.”

People with asthma and other respiratory conditions should follow their action plans, she said.

Rain expected over area on Monday

The mercury stayed above 24 degrees overnight in Adelaide after a top of 43 degrees on Sunday.

Senior forecaster Brett Gage said the city could expect showers in the morning, with more rain and a possible thunderstorm in the afternoon, when a wind change is also expected to come through.

Horses and sheep sheltering at Mount Barker during the fire. Photo: ABC News: David Frearson

Mr Stark said the wind change would “provide some challenges”, but the rain was welcome.

“There’s a little bit of rain moving across the Yorke Peninsula and the Eyre Peninsula this morning and that will be of some benefit as well,” he said.

“It’s as much about the lowering temperatures and the increasing humidity as the day goes on, which will be great,” he said.

Up to 80mm of rain is forecast for South Australia’s northern agricultural area on Monday, along with possible heavy falls and flooding in the north-west pastoral area.

Man charged over Clarendon fire

A 60-year-old Hallett Cove man has been charged with causing a bushfire after police allegedly caught him leaving the scene of a separate bushfire at Clarendon on Sunday.

Police said the fire had just started when patrols spotted a man speeding away from the scene in his car on Piggott Range Road about 6pm.

When they stopped the car, the 60-year-old driver was breath-tested, allegedly returning a blood-alcohol reading of 0.145 per cent.

He has been charged with causing a bushfire, driving unlicensed, engaging in a police pursuit, drink driving, and defacing number plates.

Police are investigating if there are any possible links between this fire and others in the area.

The Hallett Cove man was denied bail and will face Christies Beach Magistrates Court on Monday.

-ABC