Horsham Fire Brigade captain Mathew Walsh said a sub-station was believed to have overloaded in the east side of the building.

He said the building filled with smoke and forced staff and customers to evacuate.

The fire brigade, Powercor and police attended.

Safeway staff member Ema Iredell, 17, said people first noticed something was wrong at 8.50pm.

"All the power just went out,'' she said.

"The lights just flickered.

"There was heaps of smoke from the back of the building.

"We were all lost and looking at each other. Then one of the boys said it was a fire.''

Ema said the staff and customers calmly evacuated to the supermarket carpark.

Mr Walsh said the investigation would continue.

Click here for more photos of the Horsham Safeway evacuation.