A dozen illegal waste sites have been found in the city's northern suburbs during joint inspections involving the police, environment watchdog and firefighters, WorkSafe business operations boss Marnie Williams said on Friday.

"Four of these premises were occupied by Bradbury Industrial Services," she told the recycling and waste management parliamentary inquiry.

The illegal dumps were found across Craigieburn, Campbellfield and Epping.

She said the sites did not have a manifest of the dangerous goods stored, no emergency plans and there was inadequate fire protection and no spill protection.

"The premises were not fit for purpose," she noted.

The parliamentary inquiry was prompted after a string of toxic fires in Melbourne, including one at a Campbellfield factory in March that left a worker in hospital.

That site was subject to 37 Worksafe inspections prior to the blaze, and 21 notifications were issued at the site, Ms Williams said.

AAP has made repeated attempts to contact Bradbury without success.

The inquiry continues.

Australian Associated Press