Sydney (AFP) - A film about children with same-sex parents has been barred from being shown in public schools throughout Australia's most populous state, despite the documentary makers insisting students will benefit.

"Gayby Baby", which chronicles four children growing up with gay parents, was due to be shown in high schools in New South Wales as part of the student-led Wear it Purple initiative on Friday, which promotes diversity and inclusiveness.

But after front-page coverage in the Sydney Daily Telegraph this week, state Education Minister Andrew Piccoli pulled the plug, at least in school hours.

"I have directed the Department of Education to ensure the film is not shown during school hours," he said, in a move backed by state Premier Mike Baird.

"I understand the intent of that is to provide an example of tolerance and that's something I absolutely support," Baird told reporters.

"Should it be in class time? No, I don't think so. Should it be optional? Yes, I do think so."

The Telegraph reported some parents were angry their daughters had to watch the film, rated PG, or not recommended for under 15 years, but the documentary's director Maya Newell, a former student at one of the schools due to screen it, said the reaction was overblown.

"We firmly believe our film has positive benefits for all students and we're committed to supporting the schools who are celebrating Wear it Purple Day," she said on Facebook.

"Creating inclusive classroom and valuing family diversity promotes student wellbeing and acceptance of difference.

"There is no place for bullying, homophobia or discrimination in Australian public schools."

The largely crowd-funded film, which had two sold-out screenings at the Sydney Film Festival this year, is due to open in cinemas next week.

Wear it Purple Day founder Katherine Hudson told the Sydney Morning Herald she could understand the film being banned if it showed "grotesque sex scenes or violence".

"But this is a film about families. Even for conservatives, this stuff would be easy to swallow," she said.

Earlier this month, a bill to legalise same-sex marriage was introduced into the Australian parliament.

But it is doomed to fail, with Prime Minister Tony Abbott's government opposed to any changes despite growing support for marriage equality.

Same-sex couples can have civil unions or register their relationships in most states across Australia, but the government does not consider them married under national law.