Victorian senator Derryn Hinch is New Zealand-born, but it is his working ties with the United States suddenly causing concern.
Senator Hinch lived and worked in the United States in the 1960s and 1970s as a journalist and still holds his US Social Security card.
That makes him eligible for a US pension, raising questions about his eligibility to sit in parliament under Section 44(i) of the Australian Constitution.
Section 44(i) renders the election of federal politicians invalid if they are "entitled to the rights or privileges of a subject or a citizen of a foreign power."
Senator Hinch says he wrote to the US Social Security Department when he was sworn into parliament last September, instructing them not to pay his pension.
He has told Sky News he does not think he has done anything wrong.
"I do not believe that I have fallen foul of the section that says, 'Are you entitled to the rights and privileges of a subject or citizen?' I am not entitled to the rights and privileges -- I get none. If you pay a Social Security tax in America for 10 years, you are then entitled, down the track, to claim a pension."
Senator Hinch has told the ABC having a US Social Security number does not entitle him to any benefits.
"Why Section 44 comes into it, as you mentioned, is it's about rights and privileges of a foreign power. Now, I don't believe a pension is a privilege, wherever you live. I can't vote in the United States. I can't do anything in the United States. I couldn't even work there without the right papers ..."
(Reporter:) "So, just on that, because that could be a privilege, you say you don't have a green card but you do have a Social Security number. That doesn't entitle you to work there?"
"No, it doesn't."
Senator Hinch says he will ask Solicitor-General Stephen Donaghue whether he needs to refer himself to the High Court.
The National Party's Barnaby Joyce and Matt Canavan, One Nation's Malcolm Roberts and former Greens Scott Ludlam and Larissa Waters are due to face the High Court in October.
Independent Nick Xenophon and the Nationals' Fiona Nash are expected to be added to the case, which will address the possible dual citizenship of all of them.
Prime Minister Malcolm Turnbull has told Channel 7 he remains optimistic those from the Coalition will be cleared of any breach of the constitution.
"Speaking for the Coalition senators and Member, which is Barnaby Joyce, of course, we are very confident that the court will confirm that they are qualified to sit in parliament. That section has never been read literally. The court has always interpreted it in a way to realise the intent of the drafters."
But Senator Hinch has told Sky News he believes the court will probably take a stronger stand on the matter than Mr Turnbull appears to think.
"I shouldn't say this, because I criticise the Prime Minister saying the High Court will do this or will do that. My opinion is that the High Court will look at this fairly traditionally, fairly harshly, and some of us will be ruled ineligible."
Constitutional-law analyst George Williams has told the National Press Club he thinks the parliamentarians face difficulty in the High Court.
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