Prime Minister Malcolm Turnbull has called former Greens senator Scott Ludlam’s failure to renounce his dual New Zealand citizenship “remarkable”.

Mr Ludlam stepped down from federal parliament on Friday after acknowledging he was ineligible to sit in the Senate after a Perth lawyer’s discovery that the Western Australian still holds New Zealand citizenship.

Under Section 44 of the Constitution, potential candidates for the Parliament of Australia are disqualified if they hold dual or plural citizenship.

“Senator Ludlam’s oversight is a pretty remarkable when you think about it. He’s been in the Senate for so long …” Mr Turnbull said during a speech on the Gold Coast on Saturday before stopping himself.

“And there it is. He’s ineligible so I assume there will have to be a count back to the High Court to produce that replacement for him.”

When asked if Mr Ludlam will have to pay back his salary from when he was first elected to the Senate, Mr Turnbull did not rule it out, saying the decision would be left to Finance Minister Mathias Cormann.

“There is a practice about that – it’ll be considered by the Minister for Finance is my recollection of the way it’s dealt with in the past, so I’ll leave that to be dealt with by him,” he said.

It’s been estimated the back pay could be in the order of $1.6 million.

It was recently revealed the Senate and the Department of Finance were seeking the repayment of salaries and other allowances received by invalidly elected former Family First senator Bob Day and former One Nation senator Rod Culleton.

The federal government ultimately waived debts owned by Mr Day, and Mr Culleton was given the option to have his debts waived too.

A spokesperson for the Greens told The New Daily that party leader Richard Di Natale was away on leave and unable to comment on Mr Ludlam’s resignation.

Mr Ludlam’s spokesperson also declined to comment.

At a press conference on Friday Mr Ludlam said he left the country with his family when he was three years old and was “naturalised” in his teenager years.

The former senator apologised “unreservedly” for his mistake and said he should have checked his citizenship status when he was first nominated for pre-selection in 2006.

However, Mr Ludlam said he didn’t have the money to pay back his salaries and allowances.

“It’s going to be millions of dollars and my total assets amount to a fast computer and some nice shoes,” he said.

Perth barrister Dr John Cameron told The Weekend Australian he exposed Mr Ludlam’s failure to renounce his dual New Zealand and Australian citizenship.

Dr Cameron said his application to the New Zealand Department of Internal Affairs to search its register of citizens was “not driven by political ideology”.

He said he later received a certificate that showed Mr Ludlam was still a citizen and sent the document to the clerk of the Senate, but out of courtesy notified Mr Ludlam with a copy.

“I did this as a citizen, not as a lawyer, with a keen interest in the Constitution,” Dr Cameron told the newspaper.

Mr Ludlam’s position will be filed by a recount of ballot papers from the 2016 election, leading to the possible election of 22-year-old Jordan Steel-John who was third on the Greens Senate ticket.