The citizenship brawl has finally come to a head this week as the High Court hears the seven cases that have fuelled confusion and uncertainty over the makeup of the 45th parliament.

Here's your quick guide to get up to speed with the seven cases.

Barnaby Joyce, Deputy Prime Minister, Member for New England (Nationals)

Facts:

The dinky-di Deputy Prime Minister revealed to Parliament his father was born in New Zealand. Despite emigrating to Australia before his son was born, NZ law made Barnaby Joyce a Kiwi by descent.

He's since renounced his NZ credentials.

Case:

Mr Joyce argues he never knew he could be in breach of section 44 of the constitution, not knowing he could be considered a dual citizen.

Because of that, his lawyers argue he should not be shown the door. The Commonwealth is also arguing the same thing.

Mr Joyce's political nemesis, Tony Windsor has joined the case. He says allowing Mr Joyce to stay in Parliament would amount to rewarding "careless candidates" who don't bother to check their family background.

Outcome:

If the High Court rules in favour of Mr Joyce, it's business as usual. But if it finds he was ineligible to be elected last year, it'll trigger a by-election in his regional NSW electorate of New England and put Malcolm Turnbull's hold on government at risk.

Deputy Prime Minister Barnaby Joyce is a Kiwi by descent. ( ABC News: Mark Moore )

Fiona Nash, Deputy Nationals Leader, Senator for NSW (Nationals)

Facts:

Senator Nash learnt she was a UK citizen by descent, because her estranged father was born in Scotland. She argued she didn't know, because she'd had minimal contact with her dad growing up — but she did understand her sisters, who were born in the UK, were dual nationals.

Case:

As with her colleagues Barnaby Joyce and Matt Canavan, Senator Nash argues she shouldn't be kicked out of Parliament because she had no way of knowing she could be a dual citizen.

Outcome:

If the High Court rules Senator Nash ineligible to sit in Parliament, there will likely be a special count of the state Senate vote and the next candidate on the list will replace her.

There would also have to be a reshuffle of the Nationals leadership and Malcolm Turnbull's ministry. But if the court finds in her favour, there'll be no change.

Fiona Nash says she didn't know her estranged father was born in Scotland. ( ABC News )

Matt Canavan, former resources minister, Senator for Queensland (Nationals)

Facts:

Senator Canavan resigned from the Turnbull ministry when he discovered he may have been an Italian citizen by descent, through his Italian-born grandparents.

Case:

First up Senator Canavan said that when he was 25 years old, his mum had filled out paperwork registering him as an Italian citizen without his knowledge.

Since then, his lawyers have argued that's irrelevant and now claim it was a retrospective change to Italian law that may have made a two-year-old Matt Canavan "Matteo".

They argue he shouldn't be punished for the laws of another country affecting his tenure in Australia's Parliament.

Outcome:

If the High Court rules Senator Canavan ineligible to sit in Parliament, there will likely be a special count of the state Senate vote and the next candidate on the list will replace him. Because the Liberal and National parties are one entity in Queensland, it will likely also throw out the division of seats between the two branches of the party.

There'll be no changes if the court finds in his favour.

Former resources minister Matt Canavan will argue his citizenship case to the High Court. ( AAP: Lukas Coch )

Malcolm Roberts, Senator for Queensland (One Nation)

Facts:

The High Court has already ruled Senator Roberts was still a British citizen at the time of the 2016 federal election, after being born in India to a Welsh father and an Australian mother.

Arguments:

Senator Roberts reckons his case is the strongest of the seven before the High Court. His lawyers argue that even though he never believed he was a British citizen, he still took the steps to ask UK authorities about his status. The fact those inquiries were almost comically misguided doesn't matter — he still did more than his counterparts.

In that respect, comparison can be drawn to the arguments of Tony Windsor against Barnaby Joyce — Senator Roberts argues the others were careless.

Outcome:

If the High Court rules Senator Roberts ineligible, there will likely be a special count of the state Senate vote and the next candidate on the list will replace him. That's likely to be Fraser Anning, whose possible entry route to the Senate was cleared last week when bankruptcy proceedings were dropped against him.

There'll be no changes if the court finds in favour of Senator Roberts.

One Nation party member and Senate crossbencher Malcolm Roberts. ( ABC News: Marco Catalano )

Nick Xenophon, NXT leader, Senator for South Australia (Nick Xenophon Team)

Facts:

Senator Xenophon said he checked whether he was considered a citizen of Greece and Cyprus, where his mother and father were born. Both inquiries suggested he wasn't. But he later discovered he held an unusual sub-class of UK citizenship from when his father's birthplace of Cyprus was a British colony.

Arguments:

Senator Xenophon argues his class of UK citizenship is almost worthless — he has no residency rights in the UK or any rights to a passport. He argues he took steps to check he wasn't a dual national, and would never have thought he'd be captured by British laws considering Cyprus is now independent.

The Commonwealth is arguing the same position.

Outcome:

Senator Xenophon has announced he's planning on quitting the Senate to run in the 2018 South Australian state election. If the High Court finds he's in the clear, there'll be a casual vacancy declared for his seat and the Nick Xenophon Team will pick his replacement.

If he's shown the door, there'll be a special recount in the state Senate vote and the next NXT candidate will likely be elected to replace him — Tim Storer.

Nick Xenophon said he looked into whether he was a citizen of another country. ( ABC News: Jed Cooper )

Scott Ludlam, former senator for WA (Greens)

Facts:

Mr Ludlam set the dual citizenship saga going when he announced he was a citizen of New Zealand, and abruptly quit Parliament. He was born across the Tasman, and became a naturalised Australian as a teenager.

Arguments:

Lawyers for Mr Ludlam are not contesting he breached the Constitution. Neither is the Commonwealth.

But, Mr Ludlam's lawyers argue all seven of the politicians before the Court should be shown the door, because it's the most simple interpretation of the Constitution.

Outcome:

Given Mr Ludlam has already quit Parliament, the High Court's decision will only effect how he's replaced. If they find he was eligible to sit in parliament, the Greens can simply pick his replacement.

If the Court rules him ineligible, there'll be a special recount in the state Senate vote, and the next Greens candidate will likely be elected — Jordon Steele-John.

Scott Ludlam abruptly quit parliament when he announced he was a New Zealand citizen. ( ABC News: James Carmody )

Larissa Waters, former senator for Queensland (Greens)

Facts:

Ms Waters joined her Greens colleague Scott Ludlam in resigning from Parliament after discovering she was a Canadian citizen.

She was born in Canada to Australian parents, but left before she'd even reached the tender age of one.

Arguments:

Lawyers for Ms Waters are not contesting she breached the Constitution. But the Commonwealth is defending her, saying she wouldn't have known a change in Canadian law made her a dual national.

But Ms Waters' lawyers are arguing for a simple interpretation of the constitution, and argue all seven of the politicians before the court should be kicked out.

Outcome:

Because Ms Waters has already quit the Senate, the High Court's decision will impact how she is replaced. If the court finds she was eligible to sit in Parliament, the Greens can pick her replacement — even being able to renominate Ms Waters to return immediately.

If the Court rules her ineligible, there'll be a special recount in the state Senate vote, and the next Greens candidate will likely be elected — former Democrats leader Andrew Bartlett.