A petrol bomb was thrown through a window of a three-bedroom unit in Main Road West at 10.15pm last Thursday, setting the lounge room floor alight and causing $5000 damage.

The unit contained cannabis, which police estimated had a street value of more than $50,000.

A man in the unit at the time put out the fire - and police allege he managed to save half of the 70 plants in the building.

Detective Senior Constable Simon Pengelly said police believed the man was cutting up the plants, preparing them for sale, when the fire happened.

"Once he realised there were fire brigade sirens coming, he buzzed off straightaway."

Police said the man fled the scene in a white car and they were trying to track him down, as well as the thrower of the firebomb. Detective Senior Constable Pengelly said the fire may have been the result of a dispute over the drugs.

"Maybe someone was not happy over what he's doing, in the sense that he's doing something illegal; or maybe they were not happy that he's receiving a portion of the cannabis - there's a lot of money."

The MFB's St Albans station manager, Steve Allen, one of the first on the scene, said two units were sent to the incident but firefighting equipment was not required.

"We just had to search the house and we found three bedrooms full of marijuana plants. No one was in the house, but someone seems to have left in a hurry," he said.

Neighbours expressed concern about the cause of the fire.

One said she hadn't heard anything on the night but was "scared" after the incident.

Another said he didn't know about the incident until the street was full of police and fire engines.

He was unaware who lived at the property.

"I have seen people coming and going from this place, but I never knew them," he said.

Information to Keilor Downs police on 9365 3348.