All federal MPs who had foreign citizenship at the time of last year's election must be disqualified from Parliament because ignorance - or "wilful blindness" - is not a valid defence, the High Court has been told.
Lawyers for former Greens senators Scott Ludlam and Larissa Waters have sought to demolish the government's argument that five of the seven MPs under a constitutional cloud should be allowed to stay in Parliament.
In their written submission to the court filed late on Thursday, the lawyers argue the Commonwealth's claims are "not supported by the text, context, history or purpose" of the constitution. The government cannot ignore the "clear wording" of the nation's founding text just because it is politically inconvenient, they say.
"Ignorance or wilful blindness ought not excuse a person from the constraints of the constitution that would disqualify a more diligent or more perceptive candidate," the submission reads.
"In complying with obligations under the constitution, negligence should never produce a more favourable result than diligence. The referrals presently before the court fall into the category of ignorance or wilful blindness."
While the government's argument - if accepted by the court - would pave the way for Ms Waters to return to the Senate, her lawyers insist she and Mr Ludlam did the right thing by immediately resigning.
In a submission filed earlier in the week, Attorney-General George Brandis argued only Mr Ludlam and One Nation senator Malcolm Roberts were wrongfully elected because they knew - or should have known - of their foreign ties.
By contrast, Deputy Prime Minister Barnaby Joyce, fellow Nationals Fiona Nash and Matt Canavan, crossbencher Nick Xenophon and Ms Waters were all completely ignorant of their dual citizenships and should be allowed to stay.
But the Greens' lawyers say there is no requirement in the constitution that a person take any deliberate or voluntary step to acquire or retain a foreign citizenship: "The disqualification is simply a question of fact: is the person a citizen of a foreign power? If so, that person is incapable of being chosen."
They point out that all seven of the referred politicians were aware they had foreign heritage but none sought professional advice about their status before the 2016 election.
The government has also expanded on its argument in the formal submissions lodged by Mr Joyce, Senator Nash and Senator Canavan, which all lean heavily on the ignorance defence.
Lawyers for Mr Joyce admit that "at all relevant times [he] was aware that his father had been born in New Zealand".
"Nonetheless, from the age of about 10, Mr Joyce believed that his father was a citizen of Australia and no other country," the submission says.
"Mr Joyce was not aware of the possibility that he might have been a citizen of New Zealand until late July 2017, at the earliest."
They argue section 44 should not "be given its unqualified literal meaning".
"It is difficult to understand how a foreign law could create, in an Australian citizen, a sense of duty of allegiance or obedience to a foreign power in the absence of any effective choice on the part of that citizen to affirm, or at least not to abandon, some form of relationship with the foreign power in question."
Senator Nash's submission reveals that she knew her sisters were British citizens, but thought that was a consequence of them being born in England. Senator Nash was born in Australia.
Senator Canavan's lawyers argue he had "at best had only a very slender connection with Italy".
"He was less than three years old when Italian citizenship was conferred on him. The legal reasons for this under Italian law were at best obscure," the submission reads.
The court has already ruled that Senator Roberts was a British citizen when he nominated last June, and he did not successfully renounce until December - five months into his tenure. It is widely believed he will be disqualified next month.
The court will hold hearings into the matter between October 10 and 12.