Both major political parties had an "unspoken agreement" to keep a lid on the ineligibility of each other's MPs to sit in the Parliament, says the lawyer who sparked the ongoing citizenship crisis.

It was digging by Perth barrister John Cameron that outed Scott Ludlam as a New Zealand citizen and prompted the Greens senator's shock resignation in July because of his dual citizenship.

The issue has now exploded into a full-blown constitutional crisis, which has seen eight MPs including Deputy Prime Minister Barnaby Joyce and Senate President Stephen Parry forced out of Parliament after discovering ties to other countries.

WA barrister John Cameron kicked off the dual citizenship crisis by questioning Scott Ludlam's eligibility. ( ABC News: Louise Merrillees )

Under the Australian Constitution, federal MPs are required to renounce any dual citizenship before nominating as candidates for election.

"It's a very serious matter, we're not talking about parking in a spot for the disabled," Mr Cameron said.

"Serious offences may well have been committed because it's an offence under our criminal code to make a false declaration to a Commonwealth officer, as has been done on the nomination forms, and that's punishable by 12 months imprisonment."

Mr Cameron has called for the establishment of a national corruption watchdog to probe issues including the citizenship of MPs, saying it would help overcome roadblocks like those he had endured in doing his research.

"That's absolutely essential," he said.

"That's why we need the independent commission on corruption because I don't believe that an investigator from the commission would come up against the stone-walling I've come up against with the [UK] Home Office."

A Liberal Party spokesman said Mr Cameron's allegations were complete rubbish. The Labor Party has been contacted for comment.

Since Mr Ludlam's resignation a number of MPs, including Mr Joyce, were ruled ineligible by the High Court to sit in Parliament — a group known as the "citizenship seven". After this ruling, the eligibility of several other MPs has come under question.

Scott Ludlam resigned in July after realising he has been ineligible under the Constitution to sit as a senator for the past nine years. ( ABC News: James Carmody )

'A can of worms that had to be opened'

Mr Cameron said he first began digging on the citizenships of Australian MPs in 2011 because he was interested in the eligibility of then prime minister Julia Gillard and opposition leader Tony Abbott.

"No, I have no regrets at all. I think it was a can of worms that had to be opened, but I don't think the credit goes to me," he said.

"If Mr Ludlam had not resigned it would have remained buried as it had been buried for another two decades."

Mr Cameron said he had no expectation of the constitutional crisis his initial digging would spark, but said he was not surprised by the number of MPs who had now been caught up in the citizenship crisis.

"I think one should take into consideration for every MP who's been identified now, there must be a handful of other MPs who have offended in the past and are receiving their pensions," he said.

A case to pay back pensions

He said it was not right that taxpayers were paying pensions to MPs who had failed to follow the law and renounce their dual citizenship.

"I think there is a case for requiring them to pay some of it back," he said.

"There was a case in NSW in the last few weeks where a barrister was struck off for acting without a practising certificate and acting without professional indemnity insurance.

"Well not only was he struck off, but he had to re-pay to all his clients what they'd paid for him for the services that he'd carried out and I don't see a distinction between the two."