It is going to come down to this: a letter in the mail that asks you for your opinion on legalising same-sex marriage.

The Government's announcement that it would gauge public sentiment on same-sex marriage via a voluntary postal plebiscite stunned constitutional law experts and raised fears about whether the outcome could be trusted.

Professor George Williams said there were experts in our country whose job it is to conduct sensitive national votes, but those experts were not the Australian Bureau of Statistics (ABS).

"It's a body that fulfils a different set of functions," he told the ABC.

"I was mystified by it. I'm still finding it hard to understand why such a sensitive, contested process wouldn't be left to the experts within government — that's what the AEC [Australian Electoral Commission] does.

The debacle with the ABS-run census has Professor Williams worried, "this could go very wrong" if the ABS has to cobble together a roll-out in a short space of time.

Attorney-General George Brandis told 7.30 on Tuesday his job was to ensure, "as many members of the Australian public participate in this great act of democratic choice".

But that may be easier said than done.

Sorry, this video has expired George Brandis predicts 'more people will vote yes'

First things first, here's why the ABS would be given the job

The Federal Government's preferred option is a national compulsory vote on same-sex marriage — but that plan has been blocked by the Senate twice, first in November and again in August.

The postal vote organised by the ABS is a back-up plan. Essentially, it will be a statistical survey of the nation — and surveys are conducted by the ABS.

This is how Finance Minister Mathias Cormann, the Acting Special Minister of State, described the postal vote: "The Treasurer will be directing the Australian statistician to request, on a voluntary basis, statistical information from all Australians on the electoral law as to their views on whether or not the law in relation to same-sex marriage should be changed to allow same-sex couples to marry."

He later told the Senate the was "very confident that we have the constitutional legal authority to make this happen".

"Under [Commonwealth] laws, the Australian statistician and the ABS can request information of all Australians on the electoral roll. Indeed, there is express provision in the ABS Act for the ABS to have access to the electoral roll."

But there could still be a legal challenge anyway

Professor Williams says he expects this to end up in the High Court because it remains unclear if the ABS has the authority to spend this sort of money — the Federal Government says it will cost $122 million.

He says the only reason the Government is pushing ahead with this tactic is because "it's the least worst option".

"Any other criteria other than the political imperative would say stop now and don't proceed down this path," Mr Williams said.

"It's not good enough as a public policy justification."

Senator Cormann says he has the authority to release $295 million for a postal vote without approval from the Senate.

But Professor Anne Twomey, a constitutional law expert from the University of Sydney, says there could be a potential legal problem given this money is meant for urgent needs.

"It has to be for some kind of emergency that was unforeseen," she said.

"Here we have an issue that has been foreseen and previously there had been allocations to it in the budget."

As well, Ms Twomey questions whether the ABS is allowed to collect information about people's opinions.

Meanwhile, the ABS can't afford to take another hammering

Envelopes could begin arriving in people's letterboxes as early as September 12 ahead of a count in November. Professor Williams says that doesn't leave the ABS much time.

"Normally we have months in the lead-up to the election to get on the roll," Professor Williams said.

"Here, a rushed poll risks disenfranchising very large numbers of young people."

He says there is not enough time to update electoral roll details, there could be issues with the post not arriving, and it could be difficult to reach expats or members of the Australian Defence Force.

Even the experienced AEC runs into these sorts of issues in compulsory votes.

"I'm quite concerned for the ABS itself … this really important government institution risks reputational damage if this goes wrong," Professor Williams said.

"I haven't seen any plausible explanation as to why it's ABS and not the AEC running it.

"At a time when people are already sceptical and jaded of democratic processes … it's not the time to be undermining that confidence further."

Ms Twomey described it as a "courageous approach" from the Government.

Bigger doesn't mean better for statistical surveys

Dr Liz Allen, a demographer from the Australian National University, described a postal ballot as "akin to a tear-out return slip that one would find in a magazine".

She says there are serious issues with bias in a voluntary poll.

"It is not a plebiscite, it doesn't capture all of Australia's view or all of eligible voters view in Australia," she said.

"And there are many exclusions, for example people overseas who may not be able to participate. There are also issues with potential tampering."

Dr Allen says random sample surveys are much more representative of Australians.

She says she does not think there have been any high-quality surveys in the past 10 years indicating Australians do not support same-sex marriage.