Two men were taken to Fiona Stanley Hospital where one is in a serious but stable condition.
A spokesman for St John said one had suffered serious burns.
Ten firefighting units remain at the property fighting the blaze, and pollution hazmat response teams are also at the scene.
Witness Michael McKinley said one of the men taken to hospital was “in a bad way”.
“I heard a guy screaming but I didn’t actually hear an explosion, there wasn’t like flames, just a lot of black smoke,” he said.
“He had … a lot of burns.
“His workmate was helping him out as best he could, but they only had a hose, and then firies and ambulance services came.”
DFES incident controller Peter Sutton said the cause of the fire was unclear.
“We don’t believe there was an explosion, at the moment we’re treating it as a hazardous materials incident for the protection of our crew,” he said. Worksafe is investigating the incident.