Hundreds of supporters of same-sex marriage campaigners rallied in Australia's two largest cities on Saturday against a plan to hold a public vote on the issue, concerned it would cause an increase in anti-gay comments.

Australia's centre-right coalition government introduced legislation to parliament last month to hold a public vote in February 2017 on whether to legalise same-sex unions.

However, those rallying in Melbourne and Sydney on Saturday called for parliament to legalise same-sex marriage without a plebiscite.

Advocates of same-sex marriage are concerned public debate around the vote would prove harmful to members of the LGBTI community.

The centre-left opposition Labor party hasn't formally announced its position on the vote but was expected to block the government's legislation with the support of independents and minor parties when parliament sits this week.

Business leaders, such as openly gay Qantas chief executive officer Alan Joyce, have criticised Prime Minister Malcolm Turnbull's push for a vote. He said on Saturday it would put the mental health of gay and lesbian Australians at risk.