The worst fears of mental health providers about effects of the same-sex marriage survey have been wildly exceeded, new figures show.

With some groups seeing double the expected rise in demand, frontline providers are now calling for a plan to deal with the predicted fallout from the announcement of the result this week.

ReachOut Australia, which spearheaded a pro-same-sex marriage campaign by five top mental health services, was forced to divert resources from other critical services to cope with a 40 per cent surge in young gay people seeking help since the survey was first announced.

ReachOut had earlier gone public with its concerns, along with the other four frontline clinical groups – Headspace, Orygen, the Black Dog Institute and Sydney University's Brain and Mind Centre – after identifying a 20 per cent rise in demand in early September.