A large fire has ripped through a mulch facility in regional South Australia, destroying a building at the site and sending smoke across Whyalla, amid another day of blistering hot weather in the state.

About 50 firefighters from the Metropolitan Fire Service (MFS) and Country Fire Service (CFS) have been battling the blaze on Iron Knob Road at Middleback Range, which has been burning since about 8am.

Water bombers were earlier called in and the MFS said crews were working to stop flames of up to 10 metres spreading to other infrastructure, including a 3,000-litre diesel tank.

The blaze had sent large amounts of smoke across Whyalla and surrounding areas, and the MFS advised locals with breathing difficulties to remain indoors and to “close windows and doors or move to another location to avoid being impacted by the smoke”.

Firefighters had been called in from Adelaide to relieve those already on the ground, who were working to establish firebreaks around the facility. Crews were expected to be at the site for several days.

The MFS said no injuries had been reported, and the cause of the blaze, as well as the estimated damage bill, were unknown.

Temperatures have climbed into the low 40s in parts of South Australia amid a severe heatwave.

Adelaide itself had endured a third consecutive day in the high 30s, ahead of more forecast scorching conditions on Monday.

The MFS said a building had been destroyed and that crews had worked hard to stop the fire spreading to a diesel tank.(Facebook: Middleback CFS)

Bureau senior meteorologist Sarah Scully said overnight temperatures had been just as uncomfortable as those during the day.

“The minimum temperature overnight has been 27 or 28 degrees or so, but again it’s hovered around 30 degrees overnight,” she told ABC News Breakfast.

“That’s two nights in a row followed by really hot days, and we’re expecting that to continue.”

The State Emergency Service rated Monday’s heatwave conditions as “severe” across most districts, but reaching “extreme” on the lower Eyre Peninsula.

A fire weather warning applied in three districts on Sunday, with extreme fire danger ratings issued for the West Coast, eastern Eyre Peninsula and lower Eyre Peninsula districts.

A concert-goer at the WOMADelaide music festival enjoys the shade.(ABC News: Daniel Litjens)