The source of a massive factory blaze that sent a large plume of toxic smoke across Melbourne's west remains unknown as nearby businesses return to the area.
The inferno was sparked by a big explosion at the Derrimut factory on Wednesday morning which sent chemical drums flying into the air.
No one was injured and it was declared under control about four hours later, with 180 firefighters tackling the flames.
The fire was still burning on Thursday afternoon, with helicopter footage showing several walls had collapsed and part of the roof caved in.
"It is going to take some time to ascertain the exact cause and we also need to gain access to the scene, which is an active fire area with lots of contaminants and structural integrity issues of the remaining building," Fire Rescue Victoria Deputy Commissioner Joshua Fischer said.
More than three million litres of water and 40,000 litres of foam were used to suppress the fire overnight, with fire water run off causing concern for local waterways.
The Environmental Protection Authority warned people and pets should avoid Cherry Creek, Anderson's Swamp and Kayes Drain until further notice.
Wastewater has been pumped away from the site and local stormwater drains blocked off to top polluted water escaping.
"There will certainly be testing waterways and understanding what's going on," the EPA's Steve Lansdell told ABC Radio Melbourne.
The authority urged people to wash surfaces and any fruit and vegetables before eating them.
One worker died and two were injured in a fire at the same building, which houses chemical blending company ACB Group and fuel producer Powerplus, in October 2023.
That fire was also sparked by an explosion.
The EPA has issued the site with two notices and inspected it nine times since that first explosion and found them compliant.
EPA and Worksafe investigations into the 2023 fire continue, while a Worksafe spokesperson on Thursday said the authority would determine further action over the most recent blaze at an appropriate time.
Drums containing kerosene, methylated spirits, methanol and other substances were stored at the site, which fuelled the blaze.
University of Melbourne chemical engineering academic Gabriel Da Silva said all smoke was toxic and chemicals from the factory were likely destroyed in the heat.
"The risk from the chemicals here has been largely providing that fire to happen, as opposed to people being exposed to compounds that are being released," the associate professor told AAP.
Western Metropolitan MPs Moira Deeming and David Ettershank questioned the management, safety and storage of toxic chemicals in Melbourne's west.
The Anti-Toxic Waste Alliance was formed after a similar factory fire in nearby West Footscray in 2018 and several other blazes at recycling plants in the same region.
Spokesperson and former Greens MP Colleen Hartland said repeated emergencies had lessened the community's faith in regulators.
"What is it about these kind of places that ... even after the death of a worker, they go on a year later to have another massive fire?" she said.
Premier Jacinta Allan suggested agencies had enough powers to investigate the lead-up to and fallout from the "deeply dangerous" fire.
"We have, in recent years, strengthened the powers, particularly of the EPA ... and this will be thoroughly investigated by those agencies," she said on Thursday.
Opposition spokeswoman Jess Wilson said the coalition was concerned the EPA was a "toothless tiger".
"We need to make sure that the EPA is actually doing it's job, it's communicating to the public about those environmental risks but more broadly has the power it actually needs," she said.
