Investigators say the bushfire, in the state's alpine region, was ignited at multiple points along the Dandongadale River flats last Friday.

"It's extremely frustrating to have critical resources tied up at a fire that's believed to have been deliberately lit and it reduces our capacity to respond to other bushfires," Incident Controller Andy Miller said on Thursday.

Up to 12 aircraft, heavy machinery including bulldozers and more than 150 firefighters have been working around the clock to contain the blaze, he said.

Weather conditions have helped crews, and the fire is expected to reach 2800 hectares in coming days.

"Strong winds and limited rainfall are forecast this weekend, so crews are preparing fall-back lines," Mr Miller said.

The deliberately-lit blaze is one of several burning across the state including the devastating Bunyip State Park fire, east of Melbourne, which was sparked by lightning on March 1.

The Bunyip fire destroyed 29 homes, 67 outbuildings and covered more than 10,000 hectares.

Anyone who may have been in the Dandongadale area and has video or dash camera footage has been urged to contact police.

Australian Associated Press