The Turnbull government is heading for a doomsday scenario in which its constitutionally controversial MPs are struck from Parliament and its postal survey on same-sex marriage is declared invalid, one of Australia's leading constitutional experts has predicted.

In a pessimistic address at the National Press Club on Wednesday, UNSW Dean of Law George Williams predicted the High Court would take a stern view of the seven MPs currently facing an eligibility probe because they were citizens of other countries.

He dismissed Prime Minister Malcolm Turnbull's confidence in the outcome as "misplaced", including the PM's prediction that Deputy Prime Minister Barnaby Joyce would be cleared by the court.

"Joyce may survive the High Court challenge, but personally I would be surprised if he does so," Professor Williams said. "It is difficult to see, if the current law is applied, that any of the seven parliamentarians who will face the High Court are likely to survive that challenge."

George Williams said he would be surprised if the High Court ruled in favour of Barnaby Joyce or the postal survey. Andrew Meares

That is because it was unclear what "reasonable steps" the MPs had taken to ensure they met the requirements, he said, unless the court decided to take a more liberal view of the law. But it was "hard to see why the High Court would fashion an exemption", he said.

Professor Williams also said he expected the same-sex marriage postal survey to be struck down. The government has bypassed Parliament using a special fund reserved for "urgent and unforeseen" matters, but given MPs have spoken publicly about the issue for so long, that would be a "tough ask" to justify in court, Professor Williams said.

"It has the appearance of a round peg in a square hole," he said. "What about this survey is urgent, except for the fact that it is necessary because of the government's own political imperatives?"

Government court submissions on the postal survey were due to be filed on Wednesday afternoon, ahead of a determination next week.

Referred to the High Court: Nationals Fiona Nash, Barnaby Joyce and Matt Canavan.

If Professor William's predictions are borne out in coming weeks, the Turnbull government would be forced back to the drawing board on same-sex marriage, as well as facing a byelection in Mr Joyce's seat of New England and deep legal questions about the legitimacy of ministerial decisions made in the past year.

"This could greatly extend the period of instability and uncertainty," he said.

Professor Williams joked he had become regarded as the "Grim Reaper" around parliamentary corridors in recent months, as MPs fell like dominos to the section 44. He lamented the "absurd" and "ridiculous nature" of the crisis, blaming politicians for failing to propose reform to the "broken" part of the constitution.

He told reporters an audit of all 226 senators and MPs could have merit once the High Court decides the fate of the seven already in trouble, but that it would in turn provoke further instability, and could land before the courts as many as 20 politicians who may be "entitled" to foreign citizenship.

The High Court will not deal with the citizenship cases until October, prolonging the uncertainty surrounding the government. Mr Turnbull has argued Mr Joyce and deputy Nationals leader Fiona Nash do not need to stand aside from cabinet in the interim because the government has legal advice suggesting they are eligible to serve - "and the court will so hold", he said.

Follow us on Facebook