For weeks, the Australian Bureau of Statistics has sought to make very clear it is not conducting a plebiscite, a poll or even a vote on same-sex marriage - it is a survey.

And from Tuesday, it will start sending survey forms to 16 million Australians, inviting them to answer a "yes" or "no" question: "Should the law be changed to allow same-sex couples to marry?"

The ABS released copies of the form on Monday, revealing a clean and simple page alongside instructions it hopes will ensure a smooth voting process. Recipients are advised to return their survey quickly - "today if you can" - using the reply-paid envelope included with the form (it will not cost you anything to vote).

Prime Minister Malcolm Turnbull said the ABS would start distributing 600,000 survey forms on Tuesday, and he encouraged all Australians to participate in the process - and reinforced his call for people to vote "yes".

"If two gay people down the road who are living together decide to get married, how does that threaten my nearly 38-year-old marriage to Lucy? Of course it doesn't," Mr Turnbull told Triple M radio.

"If lack of commitment is the problem, then people showing commitment should be a positive. I know I sound like a very conservative person when I say this, but it's very heartfelt: I sincerely believe we would be a stronger nation if more people were married and fewer were divorced."

Fairfax Media understands the first forms will be sent to people in remote locations. All Australians should receive their forms by September 25, the ABS said, and people should mail it back no later than October 27. Surveys will not be counted if they arrive after November 7, and the result will be announced on November 15.

Bracing for some creative responses to the survey, the ABS warned Australians not to include "correspondence, complaints or other communication" with their forms, for "it will not be answered". Any extraneous material inserted in the envelope - such as glitter, as some have suggested - will be destroyed, and could potentially contaminate processing machines and spoil a person's vote.

The forms are barcoded to protect against fraud, although at least one case of someone promising to sell their vote on eBay has already been spotted. An ABS spokesman warned that was a criminal offence with penalties of up to $2100 and 12 months imprisonment.

The Turnbull government was criticised from within over the postal survey on Monday, with the National Mental Health Commission releasing a harsh statement condemning the "potential negative health impacts these debates about marriage equality will have on individuals, couples and families as they are exposed to continued scrutiny and judgement".

The commission also lashed out at opponents of same-sex marriage who have argued children in same-sex families have worse health, educational and social outcomes.

"Research in this area contradicts such claims," the commission said. It also noted frankly: "The marriage equality debate is not about same sex parents having children."

Former resources minister Matt Canavan - who quit amid a cloud over his citizenship status - shot back at the government-run commission in an interview on Sky News.

"Can't we just all grow a spine and grow up?" he asked. "Let's stop being delicate little flowers."