A voluntary postal vote on same-sex marriage in Australia will be a "referendum" on freedom of speech and the controversial "Safe Schools" program, according to the Australian Christian Lobby.

The Federal Government moved this week to hold a voluntary postal vote on the issue of same-sex marriage after an effort to reopen debate on the Government's Bill to hold a compulsory plebiscite on the issue was blocked in the Senate. At least one High Court challenge to the move to hold a postal vote - focusing in particular on the Government's decision to have the Australian Bureau of Statistics conduct the survey - has already reportedly been flagged.

Lyle Shelton, managing director of the Australian Christian Lobby, said people concerned about issues of free speech or what their children are taught should vote 'no' in the postal survey.

“Who people love is not a matter for the government but the consequences of redefining marriage are matters for all Australians," he said. “There is no greater threat to freedom of conscience and freedom of speech than same-sex marriage. Gender fluid ideology is its fellow traveller and the next great ‘step forward’, according to the UK Government which redefined marriage in 2013."

Mr Shelton expressed concern with the speed of the process - with the vote scheduled to begin on 12th September - but welcomed the opportunity for all Australians to have a say on the issue. He called on parliamentarians on both sides of the debate to respect the will of the people.

People reportedly have until 24th August to apply to be on the electoral roll for the vote.