The Turnbull government must take responsibility and invest more in frontline mental health services struggling with surging demand as a result of the same-sex marriage postal survey, Opposition Leader Bill Shorten says.

A range of mental health groups are in urgent talks this week to develop a strategy to meet the increased demand, with fears the situation will worsen as the postal survey campaign continues for another two months.

Many same-sex marriage advocates wanted a free vote on the issue in Federal Parliament and opposed a public vote – whether by plebiscite or postal survey – partly because of fears about the potential damage to some people's mental health.

Mr Shorten says the development is "exactly what we feared" and Prime Minister Malcolm Turnbull must act.