The High Court had two choices — go wide or go narrow.

In the end it chose the narrow path in the interests of certainty and stability, sticking closely to the constitutional ban on foreign citizens sitting in parliament.

In short, ignorance is no defence.

Section 44(i) says any person, "who is under any acknowledgement of allegiance, obedience, or adherence to a foreign power, or is a subject or a citizen of a foreign power…" is not capable of being chosen as a member of parliament or the senate.

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The aim is to keep out those with divided loyalties.

The Government was trying to preserve quite a few of those under question, and tried to convince the court those who were Australian born had no idea they had a dual citizenship, and couldn't have a divided loyalty.

But it didn't wash.

The court found the constitution makes no mention of only operating on those who know they hold foreign citizenship.

In fact, the court took a literal approach, finding "stability requires certainty … that from the date of nomination, a candidate for election is indeed capable of being chosen to serve, and of serving in the Commonwealth Parliament".

And that's the argument that was made by Tony Windsor, the arch rival of now former deputy prime minister Barnaby Joyce. It was anticipated Mr Windsor would contest the upcoming by-election but the former member for New England has announced he will not throw his hat in the ring.

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The ruling is in line with an earlier High Court judgement from 1992 which established a regime of taking reasonable steps to renounce a foreign citizenship to become eligible to run for parliament.

But the court has made clear that rule doesn't apply to people already elected.

The case was complicated by personal circumstances of seven individuals — here's how the High Court rationalised the result.

Barnaby Joyce

Barnaby Joyce was born in Australia, specifically in Tamworth, but his father was a New Zealander.

His father became a naturalised Australian citizen in 1978, and renounced his Kiwi heritage.

Barnaby Joyce's father did not renounce his New Zealand citizenship in time. ( ABC News: Nick Haggarty )

Unfortunately for Mr Joyce junior, his father had left it too late.

That's because at the time of the former deputy prime minister's birth, his father was a New Zealander, and by descent so was he.

Barnaby Joyce of course had no idea section 44(i) applied to him.

But it's the fact of his citizenship — not the knowledge of it — that matters after today's High Court ruling, which found he was incapable of being chosen as he was a dual New Zealand citizen.

Matthew Canavan

Matthew Canavan was born in Australia, as were his parents, making his the most complicated case.

He was alleged to have Italian citizenship by descent from his grandmother after changes to Italian law which extended citizenship to descendants through both the paternal and maternal lines.

Complicated Italian citizenship laws saved Matthew Canavan from the axe. ( AAP: Mick Tsikas )

The court took heed of a special report from two Italian lawyers.

The upshot was that a person has to take many steps to activate their Italian citizenship, as Senator Canavan's mother had done, but he had not.

Essentially the court found his Italian citizenship was "potential" rather than actual.

"Senator Canavan has not applied for a declaration of Italian citizenship," the court said.

"On the evidence before the court, one cannot be satisfied that Senator Canavan was a citizen of Italy.

"The concluding section in the joint report suggests he was not.

"Given the potential for Italian citizenship by descent to extend indefinitely — generation after generation — into the public life of an adopted home, one can readily accept that the reasonable view of Italian law is that it requires taking positive steps referred to in the joint report as conditions precedent to citizenship."

And so he was spared from being disqualified.

Scott Ludlam

Former Greens senator Scott Ludlam was born in New Zealand, but didn't realise he was a dual citizen until this year.

He resigned immediately.

The court agreed with him, finding he was not capable of being elected to parliament because of the ban on foreign citizens.

Larissa Waters

Larissa Waters was the second Greens senator to resign over the citizenship issue.

Larissa Waters quickly resigned after learning of her dual citizenship. ( AAP: Mick Tsikas )

The Government argued she was validly elected because she was an Australian citizen by descent, despite being born in Canada.

But the High Court heard evidence her birth gave her automatic dual citizenship, which hadn't been renounced when she nominated and that had made her ineligible for the Senate.

She also resigned immediately when she knew of the problem.

Malcolm Roberts

The One Nation senator was born in India. But it was his father's Welsh heritage that caused his problems.

His disqualification was no surprise after the court found in an early hearing he knew there was at least a reasonable prospect he had remained a British citizen after becoming an Australian citizen.

But it wasn't his claims of knowledge after the fact that sank his case, it was the fact of his British citizenship at the time of nomination that saw him disqualified by the court.

Fiona Nash

The Nationals senator and deputy Nationals leader was also a British dual citizen, born in Australia, but with one Scottish parent.

Fiona Nash's case was one of the most cut and dried. ( ABC News: Nick Haggarty )

"Senator Nash became a citizen of the United Kingdom and colonies by descent at birth," the court noted.

When the Immigration Act 1971 came into force she acquired the right of abode in the United Kingdom, and under British law became a British citizen in 1981.

On all levels she had failed the test for eligibility without knowing she was a dual citizen.

Nick Xenophon

Before running for Parliament, Nick Xenophon thought he had renounced or taken steps to renounce all possible dual citizenship, derived from parents from Cyprus and Greece.

The Court ruled Nick Xenophon had taken appropriate steps to renounce his British citizenship. ( AAP: Mick Tsikas )

But he was referred to the High Court after it was discovered, in a twist related to when Britain annexed Cyprus, he had British Overseas Citizenship.

An expert report to the court noted the BOC doesn't even allow a person to reside in Britain, an element the court heard was at the heart of citizenship under international law.

For that reason, the court found Senator Xenophon did not have dual citizenship with another country, and was validly elected.