Tasmanian Police are treating a bushfire in a southern forest at the centre of an anti-logging protest as arson.

Environmentalist Miranda Gibson ended her tree-sit in the Tyenna Valley last Thursday, two days after the fire broke out.

She'd lived on a platform for 449 days in a campaign to end logging in native forests, attracting global attention.

Forestry Tasmania has found the fire was deliberately lit and police are treating it as an arson case.

They're appealing for anyone who may have witnessed suspicious activity in the vicinity of Muellers or Styx Roads last Tuesday afternoon to contact them.