Australia has begun a postal vote on whether to legalise same-sex marriage as a widely watched poll indicated the country would be overwhelmingly in support.

The non-compulsory ballot, which runs until the end of October, will determine whether Australia becomes the 25th country to legalise same-sex marriage.

Despite securing 70% public support in an Ipsos/Fairfax poll, the issue of same-sex marriage had faced a political deadlock, only broken last week when the High Court gave the all-clear for the vote.

The poll illustrates why parliament should simply vote to approve same-sex marriage without holding the national ballot, opposition Labour leader Bill Shorten said.

"Change in this country only ever happens when people participate in the change," Mr Shorten told reporters in Canberra.

"Please don't leave this change to other people."

The ballot of nearly 16 million people at a cost A$122 million will help Prime Minister Malcolm Turnbull unite his Liberal-National coalition, which had been in conflict over the issue throughout his two-year tenure.

Mr Turnbull had been under pressure to resolve the impasse after two previous efforts to hold a compulsory vote were rejected by the Senate, where the government is in the minority.

Frustrated progressive members said they would side with the opposition Labor Party to secure same-sex marriage if the PM could not finally resolve the issue, though some conservative MPs threatened to resign if Mr Turnbull did not stick to a public vote, threatening his one-seat majority.

The impasse was eventually resolved when the High Court ruled the government could proceed with the non-compulsory vote, without Senate approval.