Ludlam released a statement on Twitter on Friday afternoon, saying he had "recently discovered" he still held New Zealand citizenship.
It is understood Mr Cameron obtained documents not publicly available from New Zealand Authorities.
"I sent the certificate to the clerk of the Senate, however out of a courtesy a few days ago I provided Senator Ludlam with a copy of the certificate".
"Recently it was brought to my attention that I hold dual citizenship of Australia and New Zealand", Senator Ludlam said today. "This was my error, something I should have checked when I first nominated for preselection in 2016".
The deputy leader of an Australian political party has announced he is ending his nine-year career in Parliament because he has discovered he has technically never been a senator.
He settled in Australia not long before his ninth birthday, before being naturalised when he was in his mid-teens.
The Greens senator said he was unaware that he still held New Zealand citizenship, putting it down to a simple mistake.
"This is entirely on me and I should have addressed it in 2006", he said.
Senator Ludlam can either repay the money, or apply to Special Minister of State Scott Ryan to have it waived.
'Hey everyone. I'm sorry about this, but it's a thing.
Mr Ludlam's spokesperson also declined to comment.
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Greens leader Richard Di Natale said he was "devastated" by Senator Ludlam's discovery.
The ABC's election analyst Antony Green said he thought it was unlikely Mr Ludlam's salary would be clawed back.
He lauded Senator Ludlam's intelligence on issues from digital rights to homelessness.
"He will continue to be a champion of the Greens movement and a dear friend".
The Greens in WA will then recount the ballot and their third candidate, Jordon Steele-John, a 22 year old university student, will be approached to take his seat.
Mr Steele-John, who has cerebral palsy, had to give up his British citizenship to run for parliament back in 2013.
On Friday, Mr Ludlam quit after accepting he had been ineligible to be a senator since his election in 2008.
Mr Day's Home Australia group of companies has been placed into liquidation, halting construction on 207 new homes in SA, Western Australia, Queensland, Victoria and NSW.
"I did this as a citizen, not as a lawyer, with a keen interest in the Constitution", Dr Cameron, himself a dual citizen of both NZ and Australia, told The Weekend Australian.
The Department of Finance sought to have Bob Day and Rod Culleton pay back their salaries and allowances after the High Court decided they were invalidly elected for constitutional reasons.