Senior minister Simon Birmingham has denied claims his Cabinet colleagues have been urged to keep quiet and back away from campaigning in the same-sex marriage postal poll.

The postal ballot is due to begin in a matter of weeks, with the Australian Bureau of Statistics hoping to mail ballot papers to voters in September.

Those votes would be due back in early November, before a result is declared a few weeks later.

The postal ballot is a fall-back option for the Coalition, after its compulsory plebiscite was again blocked in the Senate by Labor, the Greens and members of the crossbench.

Legalising same-sex marriage has been a divisive issue for the Liberal and National parties, but the Education Minister argued he would be making his position clear.

"I will be doing everything I possibly can to encourage Australians to vote yes, to support love over fear, to show respect over intolerance," Senator Birmingham said at the South Australian Liberal party conference in Adelaide.

"We have a chance now to look to the future and that's what I hope everybody can do — look to the future in resolving this issue, look to the future in encouraging and embracing change, in recognising that we should respect all loving Australians who want to commit their lives to each other.

"I will be doing all I possibly can, without distracting from my ministerial duties, to get a yes vote out there."

Prime Minister Malcolm Turnbull has said he will vote in favour of legalising same-sex marriage, and will encourage others to do so.

But Mr Turnbull said he had many calls on his time as the leader of the nation, after calls from Opposition Leader Bill Shorten to actively campaign for the "yes" vote.

"Well, what we need is people particularly in positions of leadership — if they're going to foist this wasteful, divisive survey on Australians, then to show the courage of their convictions," Labor frontbencher Mark Butler said.

"And first and foremost, that should be seen from the Prime Minister.

"As well as, frankly, from ministers like Christopher Pyne, George Brandis and others."

Mr Turnbull led the unsuccessful Republican movement in 1999, and there are calls from some of his then colleagues for same-sex marriage advocates to learn from the mistakes of that campaign.

Former prime minister John Howard has lent his support to the "No" campaign.