Malcolm Roberts has pleaded with the High Court to let him keep his seat in parliament, arguing that he "honestly but wrongly" believed he had done everything possible to renounce his British citizenship.
The One Nation senator, in his official submission filed on Friday, believes his case is no different from that of the five other politicians facing possible disqualification who have the backing of the federal government.
His legal team claims it's irrelevant that Senator Roberts understood there was a substantial prospect that he was a British citizen at the time of nomination.
"The question is: what did he believe his citizenship to be? He believed he was Australian and that is the end of the matter," the submission says.
Nevertheless, his lawyers argue he took all reasonable steps to renounce his British citizenship - namely, emails in May and June to non-existent addresses.
He admits, however, they "have literally gone into the ether" and were ineffective under British law.
But his case goes to his "state of mind" - that he had resolved his suspicions about possible foreign citizenship by sending the emails.
Those "honest and legitimate" attempts were sufficient to disengage section 44 of the constitution, "thus he has been and remains qualified to sit in the Senate."
The submission noted that Senator Roberts' wife, "on whom he places great reliance", was suffering a serious illness between April and August 2016 and had also spent a large time in the US looking after her then-dying mother.
Last week, the High Court ruled Senator Roberts was a dual citizen when he nominated as an election candidate.
The full bench will hear his case, and those of Deputy Prime Minister Barnaby Joyce, Nationals senators Matt Canavan and Fiona Nash, crossbencher Nick Xenophon and resigned Greens senators Scott Ludlam and Larissa Waters, in Canberra from October 10.
