A dogged environmentalist and passionate adventurer, Michael Cassidy was one of almost 1200 people who were arrested and jailed during the Franklin River blockade between 1982 and 1983.

A hugely popular waterway for canoeists, kayakers and rafters, the Frankin symbolises one of the country's most renowned wilderness battles and a landmark test of Australian constitutional law.

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It starts at the western edge of the state's Central Highlands and flows to the West Coast, meeting its confluence with the Gordon River.

It was in 1978 that the Hydro-Electric Commission proposed to construct a dam on the Gordon River, which would have flooded the river.

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Pro-dam and anti-dam protests were held throughout the state and on the mainland, attracting attention from national and global identities such as renowned botanist David Bellamy and entreprenuer Dick Smith.

The anti-dam campaign stepped up with a blockade on the site on which the dam was proposed in December 1982.

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Michael Cassidy had rafted the entire length of the majestic river to attend what would become a defining historical moment for Tasmania and one to be celebrated by his son decades later, captured beautifully on film.

That film, Franklin, premiered in Tasmania suitably on Father's Day and opened in cinemas nationally on Thursday.

Oliver Cassidy said when his father passed away 10 years ago, he yearned to do something to honour him.

It was by chance in a conversation with friends after Michael had just passed away that the film came to be.

A photographer asked advice from Mr Cassidy on best to portray the journey along the Franklin, from source to sea, on film.

Mr Cassidy replied it would be best done tying in a personal story, given the river's history, and used his own as an example.

Soon enough, he was one of the film's stars.

Mr Cassidy's trip took 14 days - four of which were hauled up on the side of the river because water levels had risen to a point where it wasn't safe to get through some of the rapids.

"It's a changing river all the time," he said.

"The one that he went down and the one that I went down were very similar, but quite a different trip I'm sure in a lot of ways.

"The guides that go down all the time time will tell you that it's a different trip every time."

Mr Cassidy said people often talked about the trip down the Franklin as being a life-changing experience; a notion he was sceptical about for himself.

"But honestly I still kind of hold on to that sensation and look forward to the next time I'll be able to get out there and feel it again because it's really what coming alive is," he said.

As well as life-changing, Mr Cassidy said it was a physical challenge like no other.

He had just completed top gender-affirming surgery a few months prior to the journey and had to work swiftly to return to fitness.

"I'd have to say that everything kind of went wrong," Mr Cassidy said.

"I was really pushing myself and every injury I'd ever had flared up again."

The toll on his body culminated when a common cold landed in him in hospital and put on a drip overnight.

On the water, Mr Cassidy carried with him his father's journal of his trip and maintained his own.

"It had been eight years at that stage since he had passed so there was definitely a sense of reconnecting," he said.

"Now that it's out in the world, I have a strong series of feelings again, and to be my true full self on the screen and be welcomed in the way that I have been as a result, has been really moving."

Mr Cassidy was supported by a team of 12, which included river guides, director Kasimir Burgess, cinematographer Benjamin Bryan and a producer.

Byran shot the whole mission on a $100,000 cinema-quality camera.

"We all thought that was madness because the camera definitely wasn't going to survive, but he insisted and I'm glad he did because the footage pulled out of that was just incredible," Mr Cassidy said.

"I worked mostly with Ben and Kas who would get into these kinds of hair-raising, slippery, dangerous positions holding a huge camera, filming me do my thing.

"They were an incredible team really. I don't know where they got the stamina."

Mr Cassidy said a first whitewater trip down a river can often make a person feel safer than they should.

"For us, it wasn't until we were on that trip and a few mishaps happened that we were able to recover from that we realised this water was so much more powerful than any of us expected," he said.

Mr Cassidy said the film was initially expected to be produced on a far more modest budget than was - shot on a bunch of GoPros with little expectation other than to see what happened.

A crowdfunding campaign at the start of 2020, to which more than 790 people contributed, changed the project's trajectory and encouraged even more money to flow from other funding bondies.

"It demonstrated there was definitely an audience and support base for telling the story of the Franklin after all this time and that it was relevant to today," Mr Cassidy said.

The film draws from Mr Cassidy's family archives and other historical documentary work.

The team managed to unearth an unreleased documentary by filmmaker Tom Hayden which was shot at the time of the blockade, but never saw the light of day due to his death.

"We were able to make connections and get hold of that footage, digitise it and be able to show it to the world for the first time," Mr Cassidy said.

"The result was quite a lot of story threads that we were able to create a tapestry out of."

He said he hoped that people considered the film a story about positive change through protest.

"I hope people can realise that we can still achieve that kind of change now," Mr Cassidy said.

"One of the things that I've learned from talking to the people involved in the Franklin River campaign was that it was won and then lost several times before it was eventually won.

"There was an uncertainty at all times, but it was worth putting up the fight for something that that mattered to all of those people.

"That uncertainty is no different to anything that we're facing today.

"If the Franklin can tell us anything, then it's worth putting that effort in and there are ways of doing it that can achieve the outcomes you want."

Franklin is screening at the Star Theatre in Launceston. Screening times can be found on the film's website: https://franklinriver.movie.

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