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."