Australia's most prominent mental health advocate Patrick McGorry and the nation's peak mental health group have called for the same-sex marriage plebiscite to be dumped because of the significant harm it would do to gay and lesbian Australians.

Ahead of a meeting with Labor leader Bill Shorten on Tuesday, Professor McGorry warned a public campaign could increase the risk of self-harm and suicide in the already-vulnerable lesbian, gay, bisexual, transgender and intersex (LGBTI) community.

"Things will be said which will hurt people. Many of them are already vulnerable. There's definitely risk involved," Professor McGorry told Fairfax Media. "Anyone working in mental health would be concerned about it."

Mental Health Australia, which represents the country's peak mental health organisations, has called on the Parliament to hold a free vote on marriage equality, arguing laws protecting any part of the population from discrimination should not be contingent on a public vote.