South Australia's catastrophic fire conditions have claimed a second life in less than a week.

A day after scorching temperatures reached 46C in the Adelaide CBD and roads literally melted, authorities confirmed a person died in the Charleston area of the Adelaide Hills.

The identity of the deceased has not been made public.

It follows the death of a man in a road crash at Lameroo, which sparked a fire.

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Police said a 24-year-old man who was the sole occupant of the car collided with a tree off the Mallee Highway about 2.30pm.

As firefighters attempt to knock down several out-of-control blazes in the Adelaide Hills, a man fighting to save his home has been critically injured.

Authorities are warning the destruction from the fires burning across the state will be hard for those affected. They said 15 homes in the Adelaide Hills are have been destroyed as well as 28 buildings and 16 cars.

There's a concerted effort from Premier Steven Marshall to manage expectations for those returning.

"My fear is this is only the tip of the iceberg," he told reporters this morning. "We haven't had a chance to get back in to assess the damage.

"We know that in addition to the buildings and vehicles lost there are very significant losses in terms of livestock, animals, crops, vineyards.

"It is going to be a real scene of devastation, especially for those people in the Adelaide Hills who have been most affected."

Authorities have also confirmed that 23 firefighters and several police have suffered injuries, including one firefighter who remains in hospital in a serious condition.

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Three Country Fire Service trucks were also destroyed.

Police have confirmed they are currently checking on the welfare of 90 to 100 people across the various fire grounds.

On Friday's day of catastrophic conditions more than 100 fires broke out before midday and have since destroyed about 40,000 hectares.

The remaining fires of most concern are the blaze in the Adelaide Hills and one burning along the north coast of Kangaroo Island.

Both are uncontrolled with fire crews and water bombers working to protect assets and establish control lines.

The latest fatality comes after two volunteer firefighters tragically lost their lives while battling a firestorm southwest of Sydney on Thursday.

Geoffrey Keaton, 32, and 36-year-old Andrew O'Dwyer were remembered as committed family men on Friday.

The two have been hailed as heroes as NSW residents prepare for a day of "catastrophic" fire danger which is almost certain to raze more homes.