Fire authorities say foam bedding and "other rubber items" fuelled a fire at a South Melbourne brothel this morning.

It is expected the brothel will have to be demolished.

The blaze started before 7:00am (AEST) in the roof of the Top on Tope brothel in Tope Street.

No-one was inside at the time and damage is estimated at more then $500,000.

The cause is unclear but police will inspect a door that appeared to have been kicked in.

Fire Commander Wayne Garrard says the building contained lots of material that fuelled the blaze.

"There's a lot of combustible material in there and, you know, in the massage parlour there would have been foam bedding and quite a few other rubber items in there so it has given off quite a bit of smoke," he said.

"John" works at a factory opposite the brothel and called the fire brigade.

He says the flames were huge.

"They were pretty high at the time and big black smoke - it was just that thick," he said.

"It was incredible the smoke that was coming out of the place."