The Federal Government's same-sex marriage postal survey has copped a fair bit of criticism, and it may not even go ahead if a High Court challenge to the plan is successful.

The survey will be run by the Australian Bureau of Statistics (ABS), and will ask Australians "whether or not the law should be changed to allow same sex couples to marry".

What could happen?

If the High Court rules the Federal Government didn't have the power to order a postal survey, it could scrap the idea.

If all the I's are dotted and the T's crossed, the postal survey will go ahead.

Who is challenging the survey?

There are two groups challenging the same-sex marriage postal survey in the High Court today.

The first group is led by Tasmanian Independent MP Andrew Wilkie, PFLAG (Parents, Family and Friends of Lesbians and Gays), and Melbourne mother Felicity Marlowe.

The second group involves same-sex marriage advocates Australian Marriage Equality (AME) and Greens senator Janet Rice.

Argument one: Show me the money

First up, they reckon the cash that has been found to pay for the ABS survey hasn't been allocated properly.

Finance Minister Mathias Cormann argues he has the power to unlock funds when there is an "urgent" and "unforeseen" need.

But the challengers argue that the postal survey doesn't meet the definition of "urgent" and "unforeseen".

Given the Government earmarked millions of dollars for a compulsory plebiscite, they reckon the Government should have known a "Plan B" option like a postal vote would cost money.

Senior ministers like Peter Dutton were also talking about the possibility of a postal vote months before the budget — what the challengers say is another example of the vote not being "unforeseen".

Argument two: Statistically challenging

Argument number two is that the ABS doesn't have the power to ask the question.

The Government argues it can tell the Bureau to ask all Australians a question.

But the challengers have hit back.

They believe the Chief Statistician has been ordered to ask Australians for an opinion, rather than collecting statistics — something that is not part of the job.

Argument three: ABS vs AEC

The ABS will be posting out ballot papers to everyone on the electoral roll.

But the challengers reckon that is a bridge too far.

They argue people sign up to the electoral roll, as managed by the Australian Electoral Commission (AEC), for the purposes of taking part in "electoral matters".

And an opinion poll is not an "electoral matter", so the AEC shouldn't be allowed to share the electoral roll details with the ABS.

When will we know the result?

The High Court can take as long as it wants to hand down a decision.

But the ABS does want to start mailing out the ballot papers from the week starting September 12.

So there is added pressure for a decision before then.