An anti-logging activist who has spent more than a year up a tree in Tasmania's south has been forced down by a bushfire.

Miranda Gibson, 31, had been up the tree since December 2011, protesting against native logging in the state.

Her supporters say police advised her to come down because an uncontrolled blaze was moving quickly toward the 60-metre platform.

Ms Gibson said it was a difficult decision to evacuate, but necessary.

"The fire's only about two kilometres from where I am so it's quite close and potentially a high level of risk if it comes this way," she said.

She was greeted with hugs from supporters, including former Greens leader Bob Brown.

Ms Gibson appeared overwhelmed and took some time to compose herself before talking to the media.

The activist says she is relieved to be reunited with friends and family.

Ms Gibson did not give a lot of details about her 15-month experience but said isolation was the hardest part.

She had recently had sleepless nights, with thick smoke and flames visible from the top of her tree.

The fire is being investigated.

Forestry Tasmania's David Brown says it started after 5pm on Tuesday but it is too early to speculate about who might have been responsible.

"In terms of how the fire started, the fire is actually being investigated at the moment so I can't give any definitive answers, but it would appear that there was some suspicious activity."

It is not clear if the activist will resume her tree-sit once the fire is contained.