Fire Service senior station officer John Mansford said the fire broke out in the laundry of a spa on the fourth floor.
"I suspect that it has something to with the drier, and I suspect that it's perhaps oils mixed with towels. It's heated up and then it just keeps on heating up and spontaneously combusts.''
Mr Mansford said the fire crews did well to contain the damage.
"They forced the fire out into the staircase, rather than into the spa area, so there's minimal damage in there.''
Mr Mansford said there was no smoke damage to the other floors.
Nobody was trapped or injured in the blaze.