Nearly 200 doctors and health workers caring for Australians of diverse sexuality and gender identity have joined calls for the Turnbull government to ditch its planned plebiscite on same-sex marriage.

In less than a week, Melbourne general practitioner Vincent Cornelisse helped co-ordinate the collection of 196 signatures from healthcare professionals in every state and territory, warning Prime Minister Malcolm Turnbull that a public campaign could be damaging for vulnerable people in the community.

The letter comes as a new poll shows public support for the national vote has collapsed, with more respondents favouring a vote in Parliament.

Support for the plebiscite has fallen from 70 per cent in July to 39 per cent in Wednesday's Newspoll, with 48 per cent of respondents saying they favour a vote by members of Parliament to resolve the issue.