Documentary on the late social justice campaigner Celia Lashlie premiering this year

A documentary on the last weeks of Celia Lashlie's life will premiere at New Zealand's International Film Festival.

The social justice campaigner died of cancer three years ago - but as a labour of love, investigative journalist Amanda Millar has delivered her friend's final message.

"The very fact that she is no longer here doesn't mean that her dreams, and her work, can't be realised," Ms Millar told Newshub.

Celia Lashlie became famous for writing about raising boys - but towards the end of her life was changing focus towards domestic violence, youth suicide, and vulnerable women and children.

"She wanted the documentary to help get some of those messages across so it was more about the community taking care of its problems, rather than agencies and government," says Ms Millar.

After being diagnosed with pancreatic cancer in late 2014, Ms Lashlie teamed up with Ms Millar - who had been working on stories with her since 2001 and had since become a close friend - to make the film, titled Celia.

"She asked me to come onboard, to tell the story of the work, and follow her through the process of her last year alive. But we didn't know that in actual fact, all we were going to be left with was days."

Ms Lashlie died in February 2015 - the day she went public with her illness.

"She died so suddenly after we started filming that I was left going, 'How can I do this?'" says Ms Millar.

The centrepiece of the documentary is Ms Millar and Ms Lashlie's last interview together - filmed just two days before her death.

"I believe that Celia knew what was going on for her. She knew that this was her biggest opportunity to say what she needed to say, and probably her final opportunity," says Ms Millar.

That 90-minute interview was meant to be the beginning, but ended up forming the bulk of the film - which Millar has spent the past three and a half years putting together.

"All along - I know this kind of sounds slightly weird - but she was guiding me, I had to stay true to what she wanted," Ms Millar says.

The film will have its world premiere in Wellington at the New Zealand International Film Festival in August.

"Celia, if she knew that she was going to have a world premiere at the Embassy, that is pretty exciting - I think everyone is kind of going, 'Woah!'" says Ms Millar.

Ms Lashlie says in the film that if people attended her funeral, then her work was done - but Ms Millar is ensuring others can carry it on after her death.