Federal Labor MP David Feeney's attempt to renounce his British citizenship was not registered with UK authorities, the High Court heard as citizenship hearings were delayed amid a wait for evidence and expert availability.
The revelation from his legal counsel came on Friday as the court heard that the Greens candidate Mr Feeney beat to keep his seat in Federal Parliament may play an important role in deciding whether or not he stayed there.
The Member for Batman and ACT Labor Senator Katy Gallagher were referred in December to the High Court of Australia, sitting as the Court of Disputed Returns, over questions about whether they held dual British citizenship.
On Friday, the court in Brisbane heard the matter would be hard to move forward until Mr Feeney, whose father is from Northern Ireland, submitted his evidence.
Mr Feeney was a UK citizen by descent but claimed to have renounced in 2007 after receiving advice from the ALP legal unit detailing the steps he needed to take, but has been unable to produce any documents to prove it.
Solicitor-General Stephen Donaghue QC told the court it was hard to make any "sensible assessment" of the case until Mr Feeney made his position clearer.
Thomas O'Brien, acting for Mr Feeney, said his client's application was not registered with the UK authorities "for some reason", partly because records were not maintained back to 2007, and his team was still making inquiries in search of more factual evidence.
Mr O'Brien submitted that his client's case would come down to what steps he took to renounce and whether they were enough.
The court heard Ms Gallagher took steps to renounce her own British citizenship before she nominated but the UK Home Office requested the original documentation and the renunciation did not go through until after the 2016 election.
Two experts set to be called to give evidence disagreed on whether British authorities were under a duty to accept the initial registration or were entitled to ask for more information, the court heard.
Mr Feeney's vanquished Greens opponent in the 2016 election, Alexandra Bhathal, was granted leave to be joined as a party to the hearing, although her role was not specified.
The court heard she had an interest as a past candidate and "prospective candidate" and her legal representative could play a role as "contradictor", to test the evidence put forward by Mr Feeney.
While Ms Gallagher would likely be replaced in the Senate by Labor ticket mate David Smith, fighting off the Greens in Batman could be a bigger challenge for Labor on the back of the minor party's byelection win in the state seat of Northcote.
The hearing was the latest step in the citizenship controversy kicked off in July when Greens Senator Scott Ludlam admitted he failed to renounce his New Zealand citizenship.
A High Court hearing in October claimed the scalps of Barnaby Joyce - since re-elected at a byelection - Nationals deputy leader Fiona Nash, One Nation's Malcolm Roberts, Mr Ludlam and Queensland Greens Senator Larrisa Waters.
Mr Feeney and Ms Gallagher's hearings were adjourned to a date still to be set, among discussion of when constitutional experts would be available to testify.
Chief Justice Susan Kiefel said she hoped for the matter to be set down for further consideration in March.
