The fire started at Pickles Auctions in Bibra Lake, a suburb in Perth, about 5.15pm (7.15pm AEST).
More than 50 cars have gone up in flames as the fire spreads through the auction yard.
Firefighters were at the scene attempting to contain the blaze but were struggling to get access to the yard.
The fire had already been burning for more than an hour.
Authorities are concerned the structures holding the cars may crumble.
Fire crews will spend much of the evening attempting to douse the blaze.