Firefighters were called to the Reservoir Road house just after midnight yesterday.

Tasmania Fire Service Launceston station officer Heath Bracey said the privately owned home had been vacant for a week before the fire.

Mr Bracey said the fire started in the bottom level of the three-storey home and had caused substantial damage.

It is being investigated by police.

The fire was just hours after suspicious fires destroyed a $450,000 house at Spreyton and a $400,000 house at Risdon Vale.

Detective Inspector Colin Riley, of Bellerive CIB, said peak ``arson season'' ran from May to October.

He said Bridgewater had the dubious honour of the most arsons in the state - about half of those reported in the past 12 months were in Bridgewater or Rokeby, compared to 17 in the North.

``It's a cultural thing: in some areas they will do graffiti, in this suburb it's about going into houses, damaging them, and then it escalates to setting them on fire,'' he said.

Detective Inspector Riley said arson was a difficult crime to solve because it left very little forensic evidence.

Bellerive CIB has the state's only dedicated anti-arson taskforce, Taskforce Icarus, which aims to break that culture.

It uses the standing Housing Tasmania reward of up to $10,000 for information that leads to the conviction of an arsonist to encourage witnesses to come forward.

In the past three years they have handed out seven rewards of between $2000 and $6500.

Detective Inspector Riley said houses targeted by arsonists were invariably vacant, often publicly owned, and had often been used as a hangout before being torched.

He asked residents to report any vacant house that appeared to have been entered or damaged.