Victorian Police believe serial arsonists are responsible for up to six deliberately lit fires around Melbourne's north-west in recent days.
Last night, fire crews were called to an industrial estate at Sunshine North, where they found three separate fires.
One fire was burning in stacks of lumber, while another destroyed two hectares of grassland along a train line.
In the third fire, two cars were destroyed.
Fire officials and police suspect the same people are responsible for lighting a grass fire on Monday night, as well as other fires in the area.
A number of fires at Wallan yesterday are also believed to have been deliberately lit.
Acting Premier Peter Ryan says the public needs to speak up if they see anyone acting suspiciously.
"So for everybody concerned, we need to make sure if you do see or hear anything that might be reflective of someone undertaking in this stupid course of conduct, contact the police as a matter of urgency," he said.
"It is simply unbelievable that we have idiots out there who are looking to pursue this style of activity.
"I mean to be lighting fires deliberately in the current environment that we as a state are facing is utterly inexcusable."