A garage on Bainfield Rd, at the northern end of the city, had caught fire at about 11.30am.
Troy Findlay was on his way to a job when he saw smoke coming out of the garage.
"I saw all the smoke and thought, "this isn't good, it's a wet day"."
He went over and made sure everyone was out safe while a woman called for emergency services.
Two fire appliances were on the scene and it appeared the fire had been extinguished by about 12pm.
Another passer-by made sure traffic was not entering the road from Queens Drive.