An Adelaide Christian school has sent a letter home to parents urging them to vote 'no' in the upcoming postal survey on same-sex marriage.

The letter came to light as groups on the other side of the debate rallied in support of same-sex marriage, including a Christian alliance in Adelaide and a major gathering in Hobart.

The three-page letter from the principal of Sunrise Christian School at Mile End begins by describing the survey as a "relatively unique event in [Australia's] democratic history".

"We are, respectfully, asking all those associated with the school community to participate and vote 'NO' in the survey," principal Shane Hosking states.

The letter, which quotes from the Bible and also refers to existing Australian laws, defends the current legal definition of marriage as a union between a man and a woman.

"It seems unlikely that any similar opportunity will arise in the future to express an opinion on what is a fundamental building block of our society," Mr Hosking said in the letter.

Speaking at a public gathering today to launch the Adelaide chapter of the group Australian Christians for Marriage Equality, religious leaders spoke against divisive messages.

"Children in schools and their parents need to be free to work this out for themselves and not to believe they're being pushed in one direction or another," AnglicareSA chief executive, the Reverend Peter Sandeman, said.

"If institutions, particularly church institutions, try and enforce a religious perspective by excluding people or seeming to threaten people, then I believe that's crossing the line."

But barrister Christopher Brohier from the group Coalition for Marriage defended the school's right to issue the letter.

"Every organisation in a democracy has a right to state its view respectfully and logically," he said.

"I have no difficulty with any organisation stating their view, just as Qantas has stated its view."

The school has been contacted for comment.

'Yes' supporters urged not to be complacent in Hobart

More than 700 people turned out for a marriage equality rally in Hobart, one of the first since the High Court gave the green light for the Government's $122 million postal survey.

Supporters of same-sex marriage gathered on Parliament Lawns urging people to vote yes in the coming postal survey.

The same-sex marriage rally in Hobart drew hundreds of people. ( ABC News )

The crowd started by chanting "the Marriage Act is not OK".

The national director of Australia Marriage Equality, Rodney Croome, is urging those supporting the 'yes' side not to be complacent.

"Polls were released showing the 'yes' vote is going down as more and more people hear the fear campaigns," he said.

"Marriage equality is an issue that affects so many families in Tasmania, so many work places, so many churches, so many sports clubs."

Rodney Croome said the 'yes' side of the debate should not assume it will win. ( ABC News )

Christian woman JoAnne Kelder told the Hobart rally the issue was about respect.

"I personally have come to the decision of 'yes' because I see it as a citizenship thing," she said.

"If you are a citizen of Australia then it's appropriate and right that you have the same rights and access to the social and legislative institutions in our country.

"I'm very aware of the fears of many Christians expressed in the public sphere but … I am also very aware and know about the deep pain and hurt and the sense of being attacked that is the lot of the LGBTQI community at the moment."

But Tasmanian Liberal senator Eric Abetz said "marriage is not to be tinkered with".

"It's not a glib line of 'love is love', it's a lot deeper [and] more concerning than that with consequences that flow to parental rights, freedom of speech, freedom of religion," he said.

"Most people I know celebrate that boys are boys and girls are girls and we should celebrate that diversity within our community and not suggest that we are genderless."