We thought it was over, but another senator has revealed they are a dual citizen.

Senate President Stephen Parry has confirmed he is a British citizen, which will trigger another recount of the 2016 election. But this one could be messy.

We asked ABC election analyst Antony Green to explain what could happen with Senator Parry's position.

Why a recount instead of a by-election?

Barnaby Joyce faces a by-election in New England because he sits in the House of Representatives, while Senator Parry sits in the Senate.

According to Green, there is legal precedent requiring a Senate recount in these situations.

"The High Court ruled in 1988 that vacancies of this type should be filled by a recount of the ballot papers," Green said.

"The procedure adopted is that the disqualified candidate is first excluded, their votes distributed to other candidates according to preferences on the excluded candidate's ballot paper, and then a full distribution of preferences to elect 12 senators is undertaken.

"In every previous case of this type, the recount elected one new senator as a replacement for the disqualified senator.

"When Family First's Bob Day was disqualified as a senator for South Australia earlier this year, the recount re-elected the 11 other senators not under challenge, with a new 12th senator, Lucy Gichuhi, elected in Mr Day's spot."

So who's going to replace Parry?

It's most likely the recount would deliver another Liberal — former senator Richard Colbeck.

Mr Colbeck was knocked down to fifth on the Liberal Senate ticket prior to the 2016 election, below Jonathan Duniam, even though he was tourism minister and Mr Duniam wasn't in parliament.

How Senate voting works In a Senate election, candidates are elected if they can secure a "quota" of votes.

In a Senate election, candidates are elected if they can secure a "quota" of votes. As successful candidates get elected by collecting a quota in a series of "counts", their surplus of votes are handed to other candidates.

As successful candidates get elected by collecting a quota in a series of "counts", their surplus of votes are handed to other candidates. If no-one in a count secures a quota, the candidate with the fewest votes is excluded and their votes are distributed based on preferences.

One party member replacing a colleague could be seen as a predictable and just outcome — if that was the end of it.

But it's not. Because of the way Senate voting works, preferences below the line make things a bit more complicated.

"Of Tasmanian Senate Liberal votes in 2016, 25 per cent were cast below the line," Green said.

"Then-senator Richard Colbeck had been demoted to fifth on the Liberal ticket but polled 4 per cent of the vote in his own right. He was defeated despite polling four times as many votes as the three Liberals elected ahead of him."

Could a recount replace a Greens senator with someone from One Nation?

Senator Parry was elected comfortably — by a margin of more than 40,000 votes over Labor senator Lisa Singh — but for those further down the count the margins were much tighter.

It took 355 counts to finally elect Greens senator Nick McKim, and the final margin over lead One Nation candidate Kate McCulloch was just 141 votes.

But according to Green, it's not the final margin that makes the difference, it's what happens further up the count.

"In the original count, Colbeck stayed in the count until the end, along with the fourth Liberal candidate, David Bushby, but ticket preferences resulted in Colbeck being defeated and his preferences electing David Bushby," Green said.

"Further preferences then elected the Greens' Nick McKim ahead of One Nation's Kate McCulloch.

"With Stephen Parry excluded from the count, David Bushby is elected as the third Liberal and then Richard Colbeck as the fourth Liberal.

"But this ordering totally scrambles the further preference count resulting in One Nation's Kate McColluch being elected ahead of Nick McKim."

So after all that, what's actually going to happen?

Although a formal recount might leave Senator McKim as the odd candidate out, Green believes his parliamentary career is not in imminent danger.

"Nick McKim was elected in 2016, he cannot be unelected by a recount caused by the disqualification of Stephen Parry," he said.

"What it means though is the court will have two new possible senators elected in the recount from the exclusion of Stephen Parry.

"The court will have to hear argument and choose between the two."

Who are they going to pick?

"The court will almost certainly choose Richard Colbeck, as he was the earlier elected candidate, and also because the election of a replacement from the same party is in line with the casual vacancy provisions of section 15 of the constitution."