Gay and lesbian groups have said the New South Wales Government is sending a message of intolerance by stopping a documentary on same-sex parenting from being shown in public schools.

The film, Gayby Baby, chronicles four children who have gay parents and how they navigate their way through life.

It was to be the centrepiece of a campaign to educate schoolchildren about same-sex families as part of Wear it Purple Day on Friday, and has prompted backlash among conservative groups and some sections of the media.

Dozens of schools across Australia were set to show the film simultaneously, including about 20 in New South Wales.

One of the schools was to be Burwood Girls High.

But an article in Sydney's Daily Telegraph newspaper stirred up controversy about the film and the New South Wales Government intervened and called off the screening.

Premier Mike Baird said the issues covered in the film did not belong in the classroom.

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

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

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The ban was said to have been prompted by complaints from parents but reports have emerged that none was actually made to the school.

Nonetheless, New South Wales Education Minister Adrian Piccoli directed all schools not to show Gayby Baby during school hours.

Victorian Premier Daniel Andrews said in a post on Facebook that the NSW Government "couldn't be more wrong".

"All kids need to know there are lots of different kinds of families, and all kids need to know that the most important thing in any family is love," he said.

"And this isn't just about what young people might think of the movie.

"I'm also worried about what they might think of themselves.

"Young people who are same-sex attracted often lead a quietly difficult life and they deserve our full support."

Banning film sends 'negative message to all young people'

Katherine Hudson, the co-founder of the Wear it Purple campaign, said by placing a ban on screening the film, the Government is taking away from the meaning of the day.

"The government has been incredibly supportive of Wear It Purple throughout all the years that it's operated but it's just very unfortunate that this has happened because in some ways it does delegitimise the day a bit ... when the Government does go out and ban things from happening," she said.

Others agree, including Cristyn Davies, an expert in child sexual development at the University of Sydney.

"I think it sends a negative message to all young people," she said.

"If this was a film about disabilities or perhaps cultural diversity or ethnicity, I wonder whether or not we would see a similar kind of response.

"Because it's about sexual diversity and gender diversity ... perhaps this is a more difficult issue for politicians to consider."

Reverend Fred Nile says 'propaganda' film will confuse children

One politician who is backing the Government's decision to block the film is the Reverend Fred Nile, the leader of the Christian Democratic Party in the New South Wales Parliament.

"Schools are where children go to study, they don't expect to see propaganda films promoting the homosexual lifestyle or same-sex relationships, especially while we're having a big debate over same-sex marriage," he said.

Mr Nile said the percentage of children who do grow up in same-sex relationships, who may find support through this film, is not high enough.

"No, that's a very, very small percentage," he said.

"The danger is you're going to confuse the great majority of children who are quite happy about the sexuality and about being part of a family."

It is not clear if any other state governments will take a similar approach to New South Wales but it does not seem likely in Victoria.

A photo put on Twitter by the Melbourne Film Festival shows Victorian Premier Daniel Andrews attending a screening of Gayby Baby with his family.

And a Federal Liberal politician said he intends to see the film and urged others to seek it out.

But Warren Entsch, who is supporting a cross-party marriage equality bill, does not believe the same-sex marriage debate should take place in schools.

"I can understand why schools, public schools say 'no'," he said.

"It's not a place in the schools. This is a debate that has to be had in the broader community but I would say the same should apply to any documentary that has a counter view."