A white powder triggered the evacuation of dozens of people at a post distributor in Abbotsford this morning.
Paramedics checked two female staff members at an Australia Post business after they were exposed to the substance, which was found to be harmless following preliminary scientific tests.
An MFB spokesman said a female worker opened the envelope and the white substance, believed to be flour, spilled onto her clothing.
"This was a deliberate hoax," the spokesman said. "These people tie up the resources that could be used elsewhere to help people that really need us."
It is believed the powder was found in an envelope containing a threatening letter created out of newspaper and magazine cuttings.