Prime Minister Bill English says American billionaire Peter Thiel did not buy his way into New Zealand citizenship, but Labour wants proof.
The co-founder of Paypal was granted citizenship in 2011, a few months after Mr Thiel gave $1 million to the Christchurch earthquake relief fund.
Mr English said citizenship could not be bought.
When he was Finance Minister, he had a meeting with Mr Thiel about investing in technology in New Zealand, but citizenship was not discussed.
Mr English could not remember the date of the meeting, which was at his Beehive office. Staff were searching his diary to establish when it happened.
Former Internal Affairs Minister Nathan Guy said yesterday he granted Mr Thiel citizenship five years ago on the advice of officials, but could not recall dealing with the application.
Mr Guy confirmed the 49-year-old was granted citizenship under a provision of the Citizenship Act that circumvented residency requirements.
Mr Thiel, who has been an adviser to US President Donald Trump, was granted citizenship on the grounds it was "in the public interest due to exceptional circumstances of a humanitarian or other nature", Mr Guy said.
Labour MP Iain Lees Galloway said he did not believe Mr Guy could have treated Mr Thiel as "just another application for citizenship".
"I don't think it's credible at all that Nathan Guy can't recall someone of the prominence and significance and wealth of Mr Thiel, who has been a subject of debate and questions in Parliament."
Mr Lees Galloway questioned what exceptional circumstances made it in the public interest to grant Mr Thiel citizenship.
The "wall of silence" from the government about Mr Thiel's application was "suspicious", he said.
The Department of Internal Affairs yesterday revealed it approved Mr Thiel's application on 30 June, 2011 - about five years after he bought a house in Auckland - but would provide no further information.
Mr Thiel, who was one of the first investors in Facebook, has since bought and sold other properties here, including in Auckland and a multi-million-dollar Wanaka lakefront estate, and invested in Kiwi tech firms.
Mr Guy declined to be interviewed by Morning Report today, saying he did not recall the application and citing privacy reasons. Internal Affairs also declined an interview request.
Mr Lees Galloway said Mr Guy was hiding behind the advice of officials. He had to own the decision and tell New Zealand why he made it.
"Ultimately the decision sits with the minister."
Normally someone must spend more than 70 percent of their time in New Zealand over five years before they can apply for citizenship.
Since becoming a New Zealand citizen, Mr Thiel would not have faced Overseas Investment Office regulatory hurdles when buying properties.