THE citizenship scandal plaguing the Australian parliament could be blow up even further with the government said to have up to five opposition MPs in the firing line.

Labor could be the next to feel the heat, with the government reportedly prepared to challenge the election of MPs they believe may not have been properly elected.

The coalition appears willing to use its numbers to challenge Labor MPs Justine Keay, Susan Lamb, Brendan O’Connor, Maria Vamvakinou and Tony Zappia on the grounds they’ve not done enough to end doubts over their citizenship, The Australian reports.

In an interview Tuesday morning, Attorney-General George Brandis accused Opposition Leader Bill Shorten of “not being honest” when it came to members of his own party whose citizenship was in question.

“Nobody can criticise the government for concealing any inconvenient facts,” Mr Brandis said.

“Mr Shorten is trying to hide behind ignorance in order to conceal the fact that some of his members may have the same difficulty.”

News of the move follows Monday’s shock revelation that Deputy Prime Minister Barnaby Joyce is a dual New Zealand citizen.

He told parliament he received advice last week from the NZ high commission that he could be a citizen of that country through his father, who emigrated to Australia in 1947. New Zealand authorities later confirmed Mr Joyce was considered a citizen by descent.

The Deputy PM is refusing to step aside before the High Court hears the matter, after the government received legal advice from the solicitor-general which suggested Mr Joyce would not be found to be disqualified under section 44 of the Australian Constitution and would not have to resign as the Member for New England.

However, the High Court will now consider his future along with that of Nationals senator Matt Canavan, former Greens Scott Ludlam and Larissa Waters, and One Nation senator Malcolm Roberts.

The outcome of Mr Joyce’s case could deal a massive blow to the coalition’s one- seat majority.

Opposition Leader Bill Shorten on Monday declined an offer by Prime Minister Malcolm Turnbull to “nominate any Labor members or senators whose circumstances may raise questions under section 44 of the constitution” so the parliament can also refer those matters to the High Court.

“The Labor Party has the strictest processes in place to ensure all candidates are compliant with the Constitution prior to their nomination for election,” Mr Shorten wrote.

“Therefore, I politely decline your offer.”

It has been suggested the expectation the government will scrutinise opposition MPs is a result of suspicions Labor encouraged its Labour allies in New Zealand to raise questions about Mr Joyce’s citizenship.

Labor has been critical of the Prime Minister’s reaction to the news his deputy could be ineligible to sit in parliament, after Mr Turnbull said Mr Joyce was “qualified to sit in the house and the High Court will so hold”.

Shadow attorney-general Mark Dreyfus said Mr Turnbull has come “perilously close to directing the High Court.

“I think the High Court justices will probably put it to one side, but they would be struck by how unusual that tone and that kind of comment is from a serving prime minister,” Mr Dreyfus told the ABC.

Attorney-General George Brandis denied the comment was directive, and that the Prime Minister was simply commenting on the government’s legal advice.

The Prime Minister is a very accomplished lawyer and has practised before the High Court and knows that better than anyone,” he told Lateline.

“What the Prime Minister was saying was expressing a view about the law which he expects the High Court will uphold, and that view was based on the solicitor-general’s advice.”

Also under scrutiny is South Australian senator Nick Xenophon who yesterday conceded he never heard back from Greek and Cypriot authorities when he wrote to renounce any possible foreign citizenship.

The Australian-born senator has declared he has never been a citizen of another country, but took cautionary moves to write to the Greek and Cypriot high commission renouncing any rights to citizenship he may have had.

Senator Xenophon is pushing for an independent audit of the eligibility of all members of parliament, and has said he would happily comply.