Green MP Ricardo Menéndez March says his partner lived with him for "several months" before the pandemic and so met the criteria for a six-month visitor visa.
Opposition MPs raised the case with immigration officials during a Parliamentary select committee meeting this morning, questioning whether there had been any special treatment.
One of the visa's criteria states an applicant must be "living together" with a New Zealander in a "genuine and stable relationship".
National Party MP Erica Stanford put a specific scenario to Immigration NZ head Greg Patchell and asked whether it would meet that requirement.
"If a couple were domiciled in two houses, in two countries, and they simply stayed with each other on holiday every now and again, would that not then meet the 'living together' instruction?" she asked.
"In the situation you've described ... it probably wouldn't meet the criteria," Patchell said.
The rules state simply sharing accommodation during holidays would not qualify as 'living together' and neither would visiting each other's separate homes.
Speaking to reporters later, Stanford said she wanted to know why then Menéndez March's partner had his visa approved in January, despite seemingly not meeting the criteria.
"The chief executive of Immigration [NZ] made it very clear that the rules around 'living together' are very, very strict," she said.
Menéndez March's partner was invited to apply for a visa on 3 December last year and a six-month visa was granted on 11 January. The Green MP flew to Mexico in December to care for his sick parents and returned with his partner in February.
He told RNZ he and his partner had followed all the correct processes and presented "a really comprehensive package" to Immigration NZ late last year.
"It's a really scrutinising and thorough process that you're put through which includes quite intimate information," Menéndez March said.
"Immigration New Zealand went through everything that we sent them and they ended up ruling that they were satisfied with that information."
He said they were able to prove they had lived together in Mexico prior to the pandemic and could explain why they were not currently.
"We lived together for several months and obviously enough for Immigration New Zealand to be satisfied," he said.
"Obviously, our goal would've been... to cohabit throughout 2020, but the pandemic has just meant that that wasn't the case."
National Party MP Chris Bishop said it was simply a matter of fairness.
"The borders are closed to New Zealand. It's incredibly difficult for New Zealanders to come home for incredibly fraught and heart-breaking reasons," Bishop said.
"We're asking the questions that New Zealanders legitimately want us to ask."
Green Party co-leader James Shaw said it was rich of the Opposition to accuse one of his MPs of wielding undue influence given National's own actions.
"It was a National Party MP [Simon Bridges] who waded in to free up 11 MIQ spots for the Wiggles," Shaw said.
"You had an MP that asserted influence and got his way and now his party are throwing aspersions at one of our MPs."
Speaking to reporters at Parliament, Immigration NZ head Greg Patchell said he was not immediately concerned about the case, but would take a closer look.
"I'm not especially [concerned], because often all you ever hear is one piece of information of a whole series of things that need to be taken into account," he said.
"I need to do the research and see what's sitting behind it ... I'll certainly ask the question."