Fire investigator Dale Lapham said the blaze at the unit caused an estimated $100,000 of damage and threatened neighbouring residences.

The fire was reported at 10pm and took more than an hour for two George Town crews to control.

Mr Lapham said any accidental causes for ignition had been ruled out.

``The unit itself was fully involved, it [the fire] had actually travelled through the adjoining roof spaces of the conjoined units,'' he said.

``It was a substantial fire, it did have potential to lose the whole block of units and there were people living in both unit 1 and unit 4, but none of those people were home at the time.''