Environmentalist Bob Brown towers in biopic The Giants

A new film about Bob Brown and the forests he's spent his life fighting to protect has been playing to sold-out theatres.

Directed and produced by the team behind the Freeman documentary, Laurence Billiet and Rachael Antony, The Giants tells the story of Brown's life, interspersed with the tale of giant trees and how they coexist in the forest.

Brown sees his participation in the film project as just another part of the environmental activism he's been known for since the 1970s.

"If it inspires one young person to become active either for social justice or the environment, it will have all been worthwhile," he told AAP.

The former national Greens party leader would be pleased early screenings in Hobart and Launceston cinemas have sold out, and The Giants has garnered positive receptions at film festivals in Perth, Adelaide and on the Gold Coast.

Brown was a leader of the fight to save Tasmania's Franklin River in the 1970s and '80s, later becoming the first openly gay member of parliament in Australia.

Now well into his eighth decade, he's still fighting as hard as ever - currently campaigning against logging in native forests across Australia, salmon farming in Tasmania and krill fishing in Antarctica.

And of course, climate change.

Brown says he's not one for looking back, but felt passing on his own experiences in the documentary might help young people who are depressed about the environment, just as he used to be.

"It's logical to get depressed when you see what's going on in the world, but we've got to get past that and get active," he said.

When asked if the film might be preaching to the converted, Brown replies the converted are in fact the majority of Australians - and he wants them to act on their beliefs.

Where does he get such energy and resilience after all this time? Part of the answer is a spiritual connection with the forest, the film reveals.

The documentary uses awe-inspiring computer generated imagery to show how the biggest trees interact with the forest environment and even with each other.

But for those motivated by the film's powerful message, Brown notes that environmental activism has changed since the 1970s.

There are harsher penalties for peaceful protest these days, he says, and organisations such as the Bob Brown Foundation face being criminalised.

The Giants is screening nationally.