More than 100 Turnbull government decisions are vulnerable to legal challenge as a result of Barnaby Joyce and Fiona Nash's dual citizenship status, with lawyers concluding there is a high likelihood the work the pair has done over the last year will end up before the courts.
The federal opposition sought urgent legal advice at the weekend after the High Court disqualified Mr Joyce and Ms Nash from Parliament - plunging the government into crisis, threatening its majority and forcing a byelection in the NSW seat of New England.
The advice from senior silk Matt Collins QC and barrister Matt Albert says Mr Joyce's and Ms Nash's ministerial decisions are now at risk under section 64 of the constitution, which requires ministers to be members of Parliament. While there is some leeway in terms of timing, Mr Joyce and Ms Nash effectively ceased to be ministers on October 20 last year - three months after they were sworn in.
Decisions that could be challenged include a range of ministerial appointments and grants, elements of the NBN regional rollout, water access entitlements and even Mr Joyce's controversial decision to move a government agency to his electorate.
Mr Albert and Mr Collins say the government - and the nation - are in uncharted legal waters, given the High Court has never been asked to make a section 64 ruling in such circumstances.
"We think that the likelihood that some purported ministerial decisions made by Joyce and Nash on or after 20 October 2016 will be challenged is high," they say in the written advice to Labor.
"We hold that view because of the combination of the unsettled nature of the law, and the significance of the decisions that Joyce and Nash will have made since 20 October 2016 having regard to the seniority and sensitivity of their portfolios.
"It is not difficult to envisage challenges being brought in respect of decisions that have had significant financial, environmental or other repercussions by corporations or persons with interests in reversing those decisions."
Researchers at the Parliamentary Library have identified at least 20 legislative instruments and 47 ministerial announcements made by Mr Joyce that could be subject to challenge. For Ms Nash, it identified eight legislative instruments and 43 ministerial announcements.
That means up to 118 decisions are open to be appealed or reversed.
That includes any decision Mr Joyce made under the Water Act, where he has significant power to determine claims for payments to water access entitlement holders.
Ms Nash's decisions on the government's rural decentralisation program, the NBN rollout, the mobile blackspot program and the Building Better Regions Fund are also vulnerable.
Constitutional expert Anne Twomey believes any decisions the pair made after finding out about their citizenship status in August would be particularly open to challenge, raising further questions about Prime Minister Malcolm Turnbull's decision to keep them in cabinet.
Attorney-General George Brandis talked down possible legal challenges on Sunday, although he conceded the government was "having a close look at that right at the moment".
"Most decisions that ministers make are in fact made by the cabinet on the recommendation of the minister. Appointments are made by the Governor-General. I think you'll find there are no legal consequences here at all," Senator Brandis said.
While Labor has indicated it is unlikely to launch any legal challenges itself, opposition frontbencher Tony Burke said "there will be vested interests who might do that".
"If you're in charge of Australia's quarantine service, there's importers or exporters who make or lose money depending on decisions you make," Mr Burke told the ABC.
"There will be a series of decisions there with vested interests ... and there [is] a whole lot of legal doubt over those decisions on the simple basis that Barnaby Joyce didn't do what Matt Canavan did. Matt Canavan turned out to have been legally in Parliament, but he at least took the precaution to step aside so that there was no risk to there being illegitimacy to his decisions."
Senator Canavan was one of the seven MPs referred to the High Court due to confusion about his Italian citizenship status, but he survived unscathed.
Senator Brandis also flagged future changes to the Citizenship Act to give certainty to people who were born overseas or who have a parent who was born overseas and want a political career.
Adam Gartrell is the health and industrial relations correspondent for the Sydney Morning Herald and The Age, based in Parliament House
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