West Australian Greens senator Scott Ludlam has resigned from the Senate effective immediately after revealing he had recently learnt he remained a New Zealand citizen despite being naturalised as an Australian citizen as a child.

Ludlam, who was born in New Zealand and moved to Australia at the age of three, says he assumed his naturalisation removed his New Zealand citizenship, but that it had recently been drawn to his attention that he remained a New Zealand citizen. Under section 44 of the constitution, anyone who “is under any acknowledgement of allegiance, obedience, or adherence to a foreign power” cannot stand for parliament.

Ludlam, the co-deputy leader who holds the communications and foreign affairs portfolios and who was first elected to the Senate in 2007, took responsibility for the oversight.

“This is entirely on me and I should have addressed it in 2006,” he said. The issue had been drawn to his attention by a “community member” who, according to Ludlam, was neither a journalist nor a political opponent. Ludlam made the decision to resign once the New Zealand High Commission had confirmed he remained a NZ citizen.

Ludlam is required to repay his salary for his period in the Senate — a sum that will exceed a million dollars. He says he intends to follow former senators Bob Day and Rod Culleton in seeking an exemption from repayment from the Special Minister of State.

Ludlam was one of the few senators with credibility in the communications sector, given his grasp of information issues, and led the fight against the Rudd government’s internet filter and the Abbott government’s mass surveillance regime. He was also an unstinting supporter of WikiLeaks and one of the few politicians to forcefully criticise the government’s failure to support Julian Assange. He was re-elected in 2014 after a special WA byelection caused by the loss of ballot papers by the Australian Electoral Commission during the 2013 federal election. The byelection became an early test of the Abbott government’s rapidly declining popularity and Ludlam’s politely phrased, but ferocious, “welcome to Western Australia” attack on Tony Abbott prior to the byelection drew huge attention.

“Thanks to you all and see you in the next life,” Ludlam said at the end of his media conference. Australian politics will miss an intelligent, articulate and credible young leader.