The Australian film Holding The Man has a firm and proselytising fan in Grammy-winning soul singer Sam Smith.

The Neil Armfield-directed feature, which was a major success at the recent Sydney Film Festival, and the Timothy Conigrave book on which it's based, "changed my life" said the British soul man in a long Instagram post.

"The most powerful thing for me was how this book captured what it's like to grow up gay and all those confusing scary and amazing moments I had coming out and realising who I was," says Smith. "And THIS FILM has captured that, and has captured Tim and Johns story so perfectly. PLEASE watch this film when you can."

Conigrave's memoir, which was adapted for the stage - to acclaim in Australia and many international productions - and then for film by Tommy Murphy, chronicles the long love affair, begun in at school in the mid 1970s with football-playing John Caleo. It's a story which takes them from Victorian school and university to Sydney's vibrant gay community and the way tragedy touched it in the 1980s and '90s.

When shown as the closing night screening at the Sydney Film Festival audiences were in tears but also very warm in response and Smith is as enthusiastic, urging his followers to not just seek out the film but spread the word.

"I long for the day when there are way more films about gay relationships, straight relationships, bisexual and transgender relationships and life stories," Smith writes. "But for now support THIS film."

While initially more constrained in his commentary on LGBT issues at the start of his career, Smith now feels not just a freedom but an obligation to say more. As he says in a Twitter post "Sorry to get so deep, but it kills me. I don't understand why more of us aren't doing something about it. I'll leave it there. But f***k."

The online response buoyed him considerably and he responded enthusiastically. "Reading through all your comments. So happy so many of you are on the same page as me. Doesn't feel so lonely right now x"