More than 130 firefighters are battling an "insane" blaze threatening buildings at a recycling centre and spewing ash and smoke across Melbourne.
Factories near the Coolaroo blaze have been evacuated, while others have been locked down, with workers told to stay indoors and turn off the air conditioning.
MFB acting deputy chief officer Ken Brown says firefighters are trying to protect an engineering building near the main fire.
"We're surrounding this fire, we're trying to contain it, he told 3AW on Wednesday.
"It's going to be an ongoing fight for some time."
Clarissa Garrow works directly across the road from the fire and she says the blaze is "insane".
"No one can get in and no one can get out," she told AAP.
"It's really bad.
"Ash and papers are flying everywhere."
This is the recycling centre's third major blaze in 2017, with another one in February needing 130 firefighters to bring it under control.
"This is a regular thing, on my first day there was a fire there," Ms Garrow said.
Abby Krtar, a worker at another nearby business, said the blaze was a big one.
"We can see smoke through the windows, ash is falling," she told AAP.
"But we're used to it."
An Watch and Act alert is in place for 18 suburbs in northern Melbourne.
"Anyone located in affected areas should take shelter indoors immediately," the emergency warning says.
"Close all exterior doors, windows and vents and ensure that heating and cooling systems are turned off.
"If you are currently driving you should close the windows in your vehicle and turn off the air conditioning."
The alert covers Broadmeadows, Campbellfield, Coburg, Coburg North, Coolaroo, Dallas, Fawkner, Glenroy, Hadfield, Jacana, Meadow Heights, Oak Park, Pascoe Vale, Pascoe Vale South, Roxburgh Park, Somerton, Strathmore and Westmeadows.© AAP 2017 Image credit: AAP Image/Stefan Postles