Labor says Barnaby Joyce owes Australians an apology over the citizenship debacle.
The former deputy prime minister and cabinet minister is now fighting for political survival in a by-election for his seat of New England after the High Court ruled him ineligible for office because he held dual citizenship.
The attorney-general's department is still finalising the cost of the court case, which involved six others including former deputy Nationals leader Fiona Nash who was also disqualified, losing her NSW Senate seat and cabinet post.
"I do think that Barnaby Joyce should apologise ... for the fact that he didn't check properly in standing for parliament," acting opposition leader Tanya Plibersek told reporters in the north Queensland city of Mackay on Tuesday.
"He should apologise for making important decisions while there was a doubt around his eligibility."
Labor has legal advice that ministerial decisions by Mr Joyce made from October 20, 2016, could be open to challenge.
The advice is based on section 64 of the constitution which provides "no minister ... shall hold office for a longer period than three months unless he is or becomes a senator or a member of the House of Representatives".
Attorney-General George Brandis is seeking advice on whether Mr Joyce's decisions would stand up to a court challenge.
However, he doubts whether there would be many at risk.
Resources Minister Matt Canavan, who stepped down from cabinet while his potential dual citizenship was tested by the High Court, is confident any challenges will withstand scrutiny.
"I note the Labor Party have decided not to challenge these decisions themselves, which to me speaks to the fact this is more about rhetoric than it is the legal reality," Senator Canavan told ABC radio on Tuesday.
Asked whether the disqualified members of parliament should pay back part of their salaries, Ms Plibersek said it was up to the government.
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She noted two other disqualified senators - Bob Day and Rod Culleton - were offered waivers of their debts.
"I suppose the government will do something similar, but it's up to the government to decide and explain the decision that they've made," she said.
South Australian Premier Jay Weatherill has sought advice about the validity of Mr Joyce's decisions.
Labor Senator Penny Wong, who also hails from SA, says the former minister "never acted in the interests of South Australia" particularly in relation to water and the Murray Darling basin.
A 17-page report compiled by the parliamentary library for Labor shows more than 100 decisions and appointments made by Mr Joyce and former cabinet colleague Fiona Nash, who was also disqualified by the court.
