Labor has received legal advice any decisions made by disqualified Turnbull government ministers are open to court challenge.
The advice from two barristers comes as the High Court prepares to hand down its judgements in the cases of seven current and former parliamentarians on Friday.
Deputy Prime Minister Barnaby Joyce, cabinet colleague Fiona Nash, former minister Matt Canavan, former Greens senators Larissa Waters and Scott Ludlam, One Nation senator Malcolm Roberts and crossbencher Nick Xenophon were referred to the court over their dual citizenship.
Australia's constitution bans anyone holding dual citizenship from sitting in parliament, in a section aimed at ensuring MPs do not hold split allegiances.
The disqualification of Mr Joyce will not only trigger a by-election in his regional NSW seat of New England, but Labor says there will also be legal questions over ministerial decisions by him and senators Nash and Canavan.
Section 64 of the constitution says "no minister of state shall hold office for a longer period than three months unless he or she becomes a senator or member of the House of Representatives".
Labor's advice is this would apply from the swearing-in date of a minister.
"Any decision made by the person in his or her purported capacity as a minister, after three months has lapsed after his or her appointment as a minister, is open to challenge," the advice from Matthew Collins and Matthew Albert says.
The government has dismissed any concerns.
The Nationals leader is widely expected to win a by-election but it will be a concern for the government, which has held on to power by one seat since the 2016 election.
Tony Windsor, a former opponent of Mr Joyce and a party to the court case, is weighing up whether to contest the seat he held from 2001 to 2013.
The government told the court the phrase "is a subject or citizen ... of a foreign power" should be seen to refer only to a person who has voluntarily obtained, or retained, that status.
Solicitor-General Stephen Donaghue argued it was reasonable that where a person had no knowledge they ever were a foreign citizen, they should not need to take any steps to renounce their citizenship.
Ms Waters and Mr Ludlam have already resigned from parliament over their dual citizenship by birth in Canada and New Zealand, respectively.
India-born One Nation senator Roberts was found by the High Court in September not to have properly renounced his UK citizenship by descent - through his Welsh father - at the time of his 2016 election nomination.
Mr Joyce and senators Canavan, Nash and Xenophon were born in Australia but were made citizens by descent.
Senator Xenophon, who found himself to have British citizenship by descent, will leave parliament whatever happens, to contest the South Australian state election in March 2018.
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