Opposition Leader Bill Shorten has deflected questions why Labor MP David Feeney - who has been unable to provide documents proving he renounced his British citizenship before being elected - shouldn't immediately resign, saying it was now a matter for the High Court.
Asked about a second Labor MP, Susan Lamb, who is also facing serious questions about her eligibility for Parliament, Mr Shorten said on Thursday he is "disgusted" that she is being pursued over dual citizenship concerns despite her difficult personal circumstances.
The citizenship imbroglio has now dragged on for more than six months and lingering doubts over a handful of MPs have ensured that the issue will remain on the agenda for some months yet.
Mr Feeney, a Victorian factional ally of Mr Shorten, was referred to the High Court late last year after failing to produce evidence that he had renounced British citizenship. Mr Feeney has not resigned - which would force a byelection for his seat - even though Liberal MP John Alexander quit last year in similar circumstances when he was unable to produce renunciation documents.
The MP for Batman confirmed on Thursday that he was still unable to find the documents, meaning more than six weeks have now passed since he first indicated he did not have documents to show he had rescinded his dual citizenship in 2007.
Mr Shorten said Mr Feeney could best explain his efforts and the High Court was now dealing with the issue.
He also said there were "heaps of differences" between Mr Feeney's case and that of Liberal MP John Alexander,.
"Mr Feeney says he's taken all reasonable steps, Mr Alexander didn't say that," Mr Shorten said.
Asked about the case of Ms Lamb on Thursday, Mr Shorten said the government missed an opportunity in refusing to support Labor's move for a mass referral of in-doubt MPs late last year, which would have included the MP for the marginal Queensland seat of Longman.
"We were happy to do that but then the government voted against that. So I hope the government reconsiders its approach as we approach [the return of] Parliament," Mr Shorten said on Thursday, adding that he was open to revisiting the issue with Prime Minister Malcolm Turnbull.
"I was quite disgusted to see some of the Liberal powerbrokers pile in on [Ms Lamb]. She has explained her family circumstances. It always used to be a traditional rule in politics that families are off limits. She has explained - and I think gone to a point of some personal pain to explain - her circumstances."
Doubts have persisted over whether she did enough before the date she was officially nominated for election in 2016, the deadline set out by section 44 of the constitution, which prohibits MPs from being dual citizens.
Ms Lamb's late father was born in Scotland and she is estranged from her mother. The British government reportedly rejected the Queensland MP's renunciation application because she failed to provide her parents' marriage certificate, needed to prove she held citizenship in the first place.
On Monday, asked about her failure to retrieve the marriage certificate with the help of her mother, Ms Lamb told The Australian newspaper that her family situation was "complex".
The Labor leader said he was confident, based on legal advice, that Ms Lamb had taken adequate steps to renounce her citizenship.
The government has warned it is willing to forcibly refer Labor MPs to the High Court if they don't do so voluntarily.
