A large number of irreplaceable historical artefacts, including the first television set to arrive in Busselton, have been lost in a fire at the West Australian town's museum.

The blaze started inside the 100-year-old building — a former butter factory — around 4:00pm on Tuesday and took six firefighting units, including one from Bunbury, almost two hours to extinguish.

Busselton Historical Society president Sandra Johnston was gutted when she arrived at the scene soon after the fire began.

"It's devastating, it really is, because historical things can't be replaced," she said.

"We had old wedding dresses, we had the first television set in Busselton. It's just devastating. We cannot replace them."

A number of valuable wedding dresses were lost in the blaze. ( Supplied: Busselton Museum )

Anxious wait for full assessment

Ms Johnston said it looked like the fire started on the top floor of the building.

Busselton's first television was destroyed in the fire. ( Supplied: Busselton Museum )

"From the outside it looks like we've lost the upstairs completely," she said.

"There was nobody in the museum at the time. We are closed on Tuesday afternoons, so if we look on the bright side, that's a really good thing.

"However all the artefacts that we had upstairs will definitely be gone. I'm looking at a shell up there at the moment.

"All we can hope is that we can save the downstairs part, but I have no idea, they [firefighters] won't let me in obviously, so it will take a couple of days before we can go in."

Ms Johnston said it could be some time before the museum was able to reopen.

"The council own the building. I don't know what they're going to do about it," she said.

"I just know they're closing it and we can't go in. It'll be sealed off until we get a report as to what happened and how safe it is."

Ms Johnston said the downstairs part of the museum housed many more artefacts, a workroom and all the society's records.

Fire investigation officers are yet to determine how the blaze started.