Forest campaigner Miranda Gibson's 457-day tree-sit has been brought to a premature end courtesy of an encroaching bushfire.

The Still Wild Still Threatened campaigner has spent almost 15 months atop her 'Observer Tree', campaigning to bring an end to what she says is the logging of world heritage-nominated forests in Tasmania.

However, Ms Gibson was forced to leave as a bushfire closed to within a kilometre of her treesit on Thursday.

"Nature can be wild and unpredictable, and whilst I was able to withstand winter snow and summer heat exposed to the elements 60 metres up the Observer Tree, it is the sensible and safe decision to climb down now rather than put lives at risk with the bushfire so close," Ms Gibson said.

Ms Gibson said her campaign to stop logging of the world heritage-nomiated forests will continue despite the end of her tree-sit.

"Although it is disappointing to leave this forest whilst these precious places continue to fall to the chainsaw, I have a huge respect for the forces of nature that are in play. And I remain as dedicated as ever to standing up for Tasmania's threatened forests," she said.

The Risdon Vale fire had burned through more than 370 hectares of forested land as of Thursday afternoon, with the Tasmanian Fire Service rating the blaze at Advice level.