About 350 firefighters are on the ground in South Australia overnight battling to control a blaze threatening homes in the Adelaide Hills before the mercury rises again on Monday.

The Sampson Flat bushfire continues to burn out-of-control in the Adelaide Hills with a perimeter stretching 180 kilometres, and has blackened more than 12,000 hectares.

Twelve homes have been lost, though that number may rise above 30 as damage assessment teams move into affected areas, South Australian Premier Jay Weatherill said.

Country Fire Service (CFS) assistant chief officer Rob Sanford said crews were prepared for strong gusts through the fireground overnight.

Sorry, this video has expired The moment a water bomber releases its load

"This part of the hills is notorious for gully winds so that might create some issues for the crews overnight but that's been factored into the planning and resource allocation throughout the night," he said.

Deputy incident controller Ian Tanner told a public meeting of residents in Lobethal on Sunday night he was hopeful fire crews could stop the blaze spreading any further, but warned the danger period was not over.

"Even with all of the resources we have, we only have to miss one stump, one spot, one tree with an ember coming out of it and on a windy day, on a northerly day, this fire will continue to burn in a southerly direction," he said.

Police road blocks were earlier set up to control traffic in and out of the area, but police have allowed some residents who fled the fire to go back home.

Mr Weatherill said people would only be allowed to return with permission from authorities.

"There are parts of the firegrounds which are becoming safer than other parts of the fireground and consideration is being given to inviting people back to assist in the fire effort to protect their own homes when it's safe to do so," he said on Sunday.

"But, once again, the overarching advice we have is listen to the advice of the authorities."

Mr Weatherill said falling branches were presenting a hazard in the area and praised the 800 firefighters who have battled the blaze.

Twenty-two people, mostly firefighters, have suffered minor injuries including burns and smoke inhalation, while one person was earlier taken to the Royal Adelaide Hospital in a stable condition after a tree crushed their car at Cudlee Creek.

There were also reports the CFS said a farmer hit by a falling tree had suspected spinal injuries.

Two elderly residents from South Australia's bushfire zone were earlier declared missing, but have since been found safe and well, according to police.

A police spokesman confirmed the pair, a Kersbrook man and Cudlee Creek woman, were safe but said no other details were available.

The total extent of the fire damage is yet to be assessed, and there has been no update to the number of houses lost, with 12 confirmed to have been destroyed and fears for 20 more.

Eric Trusz from Lower Hermitage managed to save his new home, but lost his old residence on the property.

"I was involved in the Ash Wednesday [bushfires] and this was far worse, far worse," he said.

"I just feel sorry for people that have lost more than us. The only thing I'm thankful for is that nobody's lost their lives."

Water bombers, including air tankers operating out of the Edinburgh RAAF base, on Sunday continued drops over the fireground.

Islands of land surrounded by sea of scorched earth

CFS chief officer Greg Nettleton surveyed the fireground from the air alongside Mr Weatherill and said it could have been much worse.

"The number of property losses is quite low relative to the number of properties that, from what we observed, are undamaged," he said.

"There are lots of islands of unburnt areas within the total perimeter of the fire."

Mr Nettleton said crews would work to establish control lines around unburnt areas ahead of more forecast hot conditions on Wednesday.

Thermal imaging cameras will also be used for the next few weeks so hot spots can be doused before flaring up.

Police Commissioner Gary Burns said a person who was not a resident was arrested on North East Road at Tea Tree Gully after repeatedly trying to breach a control point.

He said 30 vehicles were turned away from one checkpoint and residents looking to return home should monitor the SA Alert website for further information.

"Our main priority ... is to try and return people home as soon as possible and we thank the residents for their patience because I know it's trying," he said.

"The other aspect is obviously to have a good look at and finalise what the damage bill is to this area.

"A lot of that can't be achieved until the fireground's safe."

Mr Burns said investigations were continuing into the cause of the fire, but an incinerator remained the likely cause.

Disaster assistance funds are available for those affected by the fire.

Grants of up to $700 per family and $280 per adult will be offered for essentials such as food and clothing.

Up to two weeks of emergency accommodation will also available to those unable to return to their homes.