Tasmania's fire service has modified its 'stay and defend' policy, so it can now recommend people leave their properties on days of very extreme bushfire danger.

It used to be a choice, stay and defend or leave early.

But after 173 people died in the Victorian bushfires, Tasmania's fire service is changing its policy.

The chief officer, Michael Brown, says on days of catastrophic fire conditions, people will be told to leave.

"You get to a point where leaving is by far the best option and once the fire danger rating exceeds 100 or gets into the catastrophic rating the message will be the best place to be not at your property, leave the property and go to a safer place," he said.

Mr Brown says Tasmania has had catastrophic fire conditions twice, in 1967 and October 2006 when bush fires lit up Hobart's eastern shore.

He has reminded people that if their properties are not prepared for fires, they could be judged undefendable and left to burn.