Fire crews have halted the forward spread of a blaze burning in stubble and hay bales north of Adelaide.

The Country Fire Service says it has 21 fire crews on the scene being supported by six water-bombing aircraft and farm fire fighting units.

The blazing is burning about six kilometres north of Gawler with smoke impacting roads in the area, including the Northern Expressway.

An emergency warning has been issued for local residents.

Fire crews were also battling another blaze at Midgee on Eyre Peninsula on Friday.

The two fires came as SA sweltered through a fifth day of very high temperatures, with the mercury forecast to hit 40C in Adelaide on Friday.

Extreme fire conditions were declared across eight SA regions from Eyre Peninsula to the lower southeast.

That prompted the CFS to establish regional and state command centres, put incident management teams on standby and bring in extra aerial firefighting resources.

Rising winds on Friday, ahead of a cool change, were also expected to add to the fire risk.

– AAP