There are seven justices of the High Court, but they might be hoping they had a few more on the bench to spread the workload as Parliament House keeps pumping out legal challenges.

Alongside the citizenship crisis, the latest case kicked down Capital Hill is another chapter in the same-sex plebiscite saga — a fight over the Coalition's fall-back postal ballot.

Same-sex marriage advocates were quick to announce they would ask the highest legal authority in the land to rule whether the Government had the power to demand the poll, lodging an injunction immediately.

Prime Minister Malcolm Turnbull had announced his Government would rustle up $122 million to pay for the optional ballot.

Seemingly in an effort to avoid accusations of using the Australian Electoral Commission (AEC) as a political pawn in his plan, he drafted in the Australian Bureau of Statistics (ABS) to manage the matter.

Cue #CensusFail analogies.

Treasurer Scott Morrison has directed the Australian statistician to collect the data, and Finance Minister Mathias Cormann said he had the power to appropriate funds.

Can the Government actually order this vote without legislation?

The challenge is based on the argument the Government has acted outside its "executive power" in ordering the ballot, because it has not passed legislation in Parliament to establish and fund the poll.

There are a few issues the challengers would look to argue. Key to that is to highlight that the primary function of the ABS is to collect statistical data (the hint is in the name), and raise the argument that the question of whether to legalise same-sex marriage is not a statistical analysis.

Another plank of the challenge relates to the cost of the poll, and the concern that the Government cannot just open up its wallet and hand over such a large wad of cash without parliamentary approval.

The normal process to appropriate such funds would be through legislation.

They may also seek to highlight that the cash is going to fund AEC staff being seconded to the ABS for the plebiscite, further evidence of the alleged window dressing of getting the bureau to manage the poll.

Even though the Bureau of Statistics is managing the poll, it is relying on the Electoral Commission's roll of voters to know where to send ballot papers. Can those addresses be handed over?

We had a similar vote on the anthem, Government argues

The Government maintains it has legal advice giving it the green light for the audacious bid to discharge the election promise of a plebiscite on same-sex marriage.

The Government cites a plebiscite run by the ABS decades ago to get rid of the colonial tune "God Save the Queen" in favour of a new national anthem.

That has hardened the resolve of same-sex marriage advocates, who get angry at the choice of national anthem being likened to what they describe as a basic human right.

Senator Cormann had called on his Upper House counterparts to vote for the compulsory plebiscite if they were so concerned about the optional postal ballot.

But the Senate did not agree, so it is off to the post box we go. Maybe.

And the prospect of this being done and dusted before Christmas hangs in the balance.