EMMA ALBERICI, PRESENTER: The Turnbull Government's same-sex marriage postal survey is set to be challenged in the High Court tomorrow.

Marriage equality groups will question the legality of the ABS survey, relying on the previously tested theory that the Federal Government requires parliamentary approval to spend taxpayers' money.

The hearings come just a week before the documents are due to be posted to 16 million Australians.

Jonathon Hunyor is the lawyer representing independent MP Andrew Wilkie, Felicity Marlowe from Rainbow Families and the advocacy group Parents and Friends of Lesbians and Gays and he joins me now from Melbourne.

Jonathon Hunyor, welcome to Lateline.

JONATHON HUNYOR, LAWYER: Good evening, Emma.

EMMA ALBERICI: So what is the argument that you will be bringing to the High Court? And what's the law you are relying on?

JONATHON HUNYOR: Well, look, there's a number of arguments. The fundamental argument that we will be raising is that the power that the Finance Minister has purported to use to fund the postal survey, or postal vote is unconstitutional.

There's a provision in the Appropriation Act which passed this year, as it usually does as part of the May budget, that gives an advance to the Finance Minister of currently $295 million to use in urgent, unforeseen cases or circumstances.

We're arguing that that provision is itself unconstitutional, because it effectively circumvents the basic constitutional rule that it's for Parliament to appropriate; for Parliament to authorise government spending.

The second argument we run is that, even if that power is itself valid, the Finance Minister has exercised it in this case unlawfully, because there's nothing urgent about having a postal vote on same-sex marriage.

Lots of people want to get it resolved very quickly, but the Government has set a timetable that makes it urgent for them. That doesn't mean it's urgent within the meaning of that provision; and nor is it unforeseen, which is the other limb of that test.

Our third argument is that the Government, by co-opting the Australian Bureau of Statistics into the process, is again acting unlawfully because at its heart this is not a statistical process. In reality, we say, it is a postal vote, which is how the Government has described it. And so we will be asking the High Court to find that it's not valid, then, beyond power for the ABS to get involved.

And finally: a similar argument in relation to the Australian Electoral Commission. What is happening here is a vote, but it's not an election. And the Electoral Commission's powers are limited to electoral matters. So we say: this falls beyond that.

So, for those reasons, we say that the Government can't go ahead with this process.

EMMA ALBERICI: The Government will challenge whether your clients have a legitimate special interest in this case: technically speaking, that they have standing to allow them to bring the case.

JONATHON HUNYOR: That is right. The laws in relation to standing in Australia are historically fairly narrow. You need to show that you have got a special interest in the proceedings.

We say that all of our clients do. For Andrew Wilkie, as a parliamentarian he has got a particular interest in making sure that government processes, parliamentary processes, aren't subverted and that the constitutional rules around appropriation are followed.

For Felicity Marlowe, she is in a same-sex couple herself with kids and so the issue bears particularly on her and her kids. They have already, as they feared, been feeling the brunt of some of the homophobic comments and the judgement that she feels society passes on the validity of her relationship.

And for PFLAG: that's a group that is there to support parents and friends of lesbians and gays and rainbow families. And so for them it's very much part of their fundamental business and their interests.

So, for those reasons, we do think we fall within standing. But that's obviously a fundamental question the High Court will have to answer.

EMMA ALBERICI: Now, are you relying on any specific precedents in bringing your case? Are there examples where the High Court has overruled the government and denied it the right to spend taxpayers' money?

Oh, look, there's a range of cases that have considered these sorts of issue. There was the case involving school-based chaplains: the Williams case, for example. There's a range of cases that have looked at these issues.

And this in effect gives us a chance to test out the limits of those cases and to determine whether or not, in the way the Government has gone about it in this case, whether it falls within the limits of their power.

EMMA ALBERICI: So you're talking about the school chaplaincy program?

JONATHON HUNYOR: That's right. That's right.

EMMA ALBERICI: That was challenged in the High Court?

JONATHON HUNYOR: That is right. It was.

EMMA ALBERICI: And the Government was seen not to be able to spend that money on that program?

JONATHON HUNYOR: That is correct, yes.

EMMA ALBERICI: Because the Parliament didn't approve it: is that correct?

JONATHON HUNYOR: Yes, that is right.

EMMA ALBERICI: Is there a wider principle you're trying to establish here, beyond the implications for same-sex marriage?

JONATHON HUNYOR: Look, absolutely. The case is really, at its heart, about how government power is exercised and about how our system of democracy works.

The fundamental point that, really, we make in this case and that our clients make is that it should be for Parliament to make these decisions. Now, Parliaments are voted in by the people to make difficult decisions.

And if we look at what is going across Australia: in New South Wales at the moment, the issue of euthanasia is being debated there. And that's not something that is going to be the subject of a postal survey: that is something that Parliament is going to decide and it's an issue that obviously raises significant moral and ethical questions.

I know in the Northern Territory, access to RU486 and abortion laws have been the subject of recent parliamentary consideration. And you just need to look at the history of Australian laws dealing with human rights issues and significant issues that Australians expect our Parliament to be able to deal with.

We never go through this sort of process and we think it's not a good way to run government.

EMMA ALBERICI: What gives you the confidence to believe your side has a good chance of beating the Government on this and convincing those seven judges in the High Court to come your way?

JONATHON HUNYOR: Oh, look, obviously we've had a look through the authorities and whether the law lies. We've got advice. We've got a team of fantastic barristers working on it. We wouldn't be running it if we didn't think it was a case we could win. So we will just have to see what the High Court has to say.

EMMA ALBERICI: But isn't it entirely possible that the High Court will give weight to the Government's claim that it became urgent that they honour this election promise to put same-sex marriage to a public vote?

JONATHON HUNYOR: Well, look, you know, I can't pre-empt what the High Court is going to find. Obviously the Government is going to argue that there is an urgency to this.

Our argument is a pretty simple one: there's no urgency in a case like this and urgency should mean where there is a natural disaster or something needs to happen immediately.

I mean, if you look at something like the Northern Territory Emergency Response from 2007: that was something where the government decided there was an emergency, something urgent. They took it back to Parliament. They got a special appropriation to spend money on that.

This regime, the postal survey to be conducted by the ABS, was actually announced during a sitting week: the first of two sitting weeks. So our argument is: well, it was absolutely within even the Government's own timetable that it could bring it back to Parliament.

They didn't do that because they knew it would fail, as the plebiscite bill had done. They're trying to get around that and we say that they can't do that consistent with the constitution and consistent with the limited power that they do have to spend money in these sorts of circumstances.

EMMA ALBERICI: When is the court likely to make a decision? And what happens if such a decision hasn't come before September 12th, when those postal surveys are supposed to be mailed out?

JONATHON HUNYOR: Look, we don't know when the High Court will decide. It's at least possible that they will give a decision, if not their reasons, at the end of the hearing on Wednesday.

But the Government has agreed that they won't start sending anything out until 12th, so it is entirely possible the court will take that time to further consider their decision; or, indeed, they may decide they need longer; in which case we need to either get the Government to agree to postpone things or we may need to make application to restrain them from going ahead.

But we will cross that bridge when we come to it. Right now, we're just focused on running the case tomorrow and on Wednesday.

EMMA ALBERICI: And does it need to be a majority of the seven judges or a unanimous decision?

JONATHON HUNYOR: No, a majority will do just fine.

EMMA ALBERICI: In the event that you win and it's a case that the postal survey is thrown out: do you consider it inevitable that the Government will have to move to a free vote in the Parliament?

JONATHON HUNYOR: Look, that's certainly what we would hope. We think it's for Parliament to do its job and we want it to get on with it and to end discrimination against same-sex couples and give them the right to marry as do other Australians.

EMMA ALBERICI: And just briefly: there is also another case being run, seeking the same outcome: for the postal survey not to go ahead. The other case is being run by Senator Janet Rice of the Greens, along with Australian Marriage Equality. Why are these two cases being held concurrently?

JONATHON HUNYOR: Oh, look, there are two cases being brought because there are different groups within the community, all who have a shared interest in stopping a process that they think is unnecessary, is harmful and divisive, is incredibly expensive.

So the two cases have been brought and the arguments that are being made are different in the cases. Our case raises the constitutional questions. The AME and Rice case raises some different issues. So it's certainly a case where...

EMMA ALBERICI: What are the issues that they're raising, briefly, that are different to yours?

JONATHON HUNYOR: Sure. One of the issues they raise is the nature of the power under the Appropriation Act, that they say is limited to funding for the ordinary business of government. And they are arguing that this is not within the ordinary business of government. This is an extraordinary process; a very expensive and complicated one.

So they say that it falls beyond the scope of the power in the Appropriation Act that the Government is purporting to act under.

So it's the sort of case where more legal minds is better because it's a complicated case to some extent - well, to a large extent it's unchartered waters. And so having both cases there is really useful in getting to the bottom of some difficult issues.

EMMA ALBERICI: We will watch with interest. Jonathon Hunyor, thank you very much for your time this evening.

JONATHON HUNYOR: Thanks, Emma. Pleasure.