A major freeway is shut and thousands of residents have been told to stay indoors as toxic smoke from a “significant” factory fire billows over parts of Melbourne.

Metal drums filled with chemicals were thrown into the air from deep within the blaze, posing a danger to the 180 firefighters racing to bring it under control on Wednesday afternoon.

A large explosion triggered the blaze in the chemical factory about 11am. It then engulfed the building in an industrial park at Swann Drive, Derrimut, west of the CBD.

Victoria Police said there were no reports of injuries and it was believed everyone inside the factory had been evacuated safely.

Fire Rescue Victoria said hazardous material and unknown toxins were burning.

“It’s still burning, it’s not under control yet. We estimate that we’re going to be here a couple of days because of the fuel load in this factory,” Deputy Commissioner Michelle Cowling said earlier.

Earlier, during intense explosions at the chemical fire in Derrimut, I captured this drum flying through the air – one of many sent sky-high. Hate to think where they landed. Thankfully it seems to slowly easing now. #melbourne @7NewsMelbourne pic.twitter.com/d4BEOqHIxU

— Paul Dowsley (@paul_dowsley) July 10, 2024

Country Fire Authority crews were brought into to assist FRV crews, with teams working in 45-minute shifts to quell the blaze. Specialist hazmat and scientific advisors are also on site, as well as the Environment Protection Authority.

“This is a significant fire … it is quite dangerous. It is a chemical factory, so we are urging people – please stay away from the area,” Cowling said.

“We don’t know what’s in that smoke, smoke is toxic and will cause damage.”

She said the blaze was Melbourne’s most significant in several years.

“We’ve got embers burning, fire burning … Even with our aerial appliances that are on site and specialist hazmat people on site, we can’t get in close enough to cool this fire down,” she said.

The fire was declared under control shortly before 4pm Wednesday.

A watch and act alert remained for residents and workers in Albion, Braybrook, Brooklyn, Derrimut, Laverton North, Sunshine, Sunshine West, Tottenham and Truganina.

“Wind is blowing toxic smoke towards the east across Derrimut,” the alert stated.

“Anyone located in the area affected by smoke should take shelter indoors immediately.”

This shot from an EPA Victoria drone over the Derrimut factory fire shows the intensity of the fire. EPA is on scene, advising fire crews. Air quality advice for the public https://t.co/REKgFJwVaa @MelbourneWater @FireRescueVic @CFA_Updates pic.twitter.com/GNDSNlUV1N

— Environment Protection Authority Victoria (@VicGovEPA) July 10, 2024

The Western Freeway was also closed in both directions between the M80 Western Ring Road and Robinsons Road at Derrimut.

There was no threat to the community, but people were urged to monitor conditions.

Smoke from the blaze could be seen across Melbourne’s west. Ravenhall worker Trent Chamberlain told the ABC he could see “thick, thick black smoke” from his office about three kilometres away.

“Every few minutes, [there’s] a large fireball explosion,” he said.

“It’s still billowing out smoke, quite heavy black smoke, so it’s clearly not contained … it looks really bad.”

– with AAP