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.

LGBTI Australians are already twice as likely to have a high or very high level of psychological distress as their heterosexual peers and are at a higher risk of suicide than the general population.

MHA chief executive Frank Quinlan said the same-sex marriage debate was already having a negative impact on LGBTIQ people, especially young people, and urgent additional funding would be required for mental health services if a plebiscite were held.

"We are saying a plebiscite is unnecessary. It's the routine function of the Parliament to pass laws that remove discrimination," he said.

"Political parties of all sides change their policies to meet changing circumstances over time. We have already seen the Coalition change its super policy which it took to the election, so ... perhaps the Coalition can make this decision [to drop the plebiscite]."

The intervention from MHA, which represents dozens of organisations including Beyond Blue, Carers Australia and Lifeline, also adds weight to the Labor opposition's inclination to block the plebiscite on the grounds that it will have a significant negative health impact.

Mr Shorten and deputy leader Tanya Plibersek will today meet with major LGBTI groups in Sydney including Australian Marriage Equality, ACON and Twenty10 to gauge opinion on the plebiscite. His office says it has been inundated with correspondence from LGBTI people begging Labor to oppose the plebiscite.

Without Labor's backing, the Turnbull government lacks the numbers needed to pass the enabling legislation through the Senate.

Professor McGorry, a former Australian of the Year, acknowledged it could be harmful to allow the issue to drag on until the next election, but said a plebiscite posed "the more immediate risk" to mental health.

"The 'no' case will presumably make arguments which will be interpreted by LGBTI people as homophobic. That goes to the heart of your identity, doesn't it?" he said.

"[Homophobia] is being given a platform and it sounds like public funding is on the table to amplify the voice. They have a megaphone to do it and it will seem to be mandated in some way."

One in five gay Australians is currently experiencing depression, which is more than triple the rate of heterosexual Australians, while one in three LGBTI Australians is currently experiencing an anxiety condition, double the rate of their heterosexual counterparts.

Prime Minister Malcolm Turnbull has repeatedly dismissed concerns about the negative tone of a plebiscite campaign, insisting the debate would be conducted respectfully on both sides.

Last week, close to 200 doctors and health workers submitted a petition asking Mr Turnbull to ditch the plebiscite because of concern the debate would not live up to his expectations.