A blaze has swept through a 125-year-old former court house containing old records and valuable photos in Melbourne's north-west.

The Metropolitan Fire Brigade (MFB) said at least 50 firefighters tackled the fire at the building, which houses the Essendon Historical Society Museum, on Mount Alexander Road at Moonee Ponds about 8:30am.

The top floor of the two-storey former court house was fully ablaze, but crews brought the fire under control within an hour.

A large amount of smoke billowed from the building and people were advised to avoid the area.

The MFB said a faulty light transformer in the building's ceiling caused the fire and the damage bill would be about $450,000.

Essendon Historical Society president Bob Chalmers said he was devastated.

"We've got all our records and photos inside and we've worked very, very hard to restore the building in recent times and now it's ruined," he said. "It's heartbreaking."

Fire fighters extinguish the blaze at the heritage-listed building. ( Supplied: MFB )

Old photos could be lost in blaze

Mr Chalmers said the photo collection housed in the building was valuable.

"Mainly photos of the people and places dating well back so I don't know what state they'll be in, but it won't be very good," he said.

"We were in the process of [digitising the photos], and there are some off site probably that we can rescue but we won't know until we look into it how much we've actually lost."

The MFB said items stored in a room at the rear of the building, however, had been saved.

Kellaway Avenue is closed to traffic.