National says if it's elected, Kiwis overseas who can't find an Immigration NZ-approved lab to do a COVID-19 test won't be allowed to come home.
The party's health spokesperson Shane Reti also admitted he didn't know if implementing such a policy would require a law change.
Earlier this week National unveiled its border policy, promising a new agency to oversee efforts to keep COVID-19 out of the country and mandatory negative tests for homecoming Kiwis before they board a plane.
The Green Party said the latter would leave New Zealanders "stranded overseas, where we can't guarantee they will have access to adequate healthcare or support".
"They also make the incorrect assumption that tests are easily accessible overseas," said immigration spokesperson Golriz Ghahraman.
"We know that in many places, including the United States and even parts of Australia, testing is hard to access or very expensive. It could leave New Zealanders stranded overseas indefinitely."
Dr Reti told Newshub Nation on Saturday the party "checked where most of our arrivals come from, those points of departure, whether it's legitimate to be able to do that within 72 hours, and we think it can be done".
Most returning Kiwis are arriving from Australia, he said. Those coming from more far-flung parts of the world will have to find labs Immigration NZ has vetted and approved, or their test results might not be accepted.
"You will need to make access to that laboratory - that is an expectation."
A number of other countries require a pre-departure test for new arrivals, even coming from places like New Zealand with just a handful of active cases.
Newshub Nation host Simon Shepherd asked Dr Reti if it was even legal to block Kiwis from coming home under the current law.
"If you can't meet these criteria, then you won't be able to board," said Dr Reti. "Whether we'll need to pass legislation, that's something Gerry Brownlee... will need to look at and part of what the Border Protection Agency will need to figure out...
"We're not concerned at this point that we're breaching any legislation that would be a concern, that would disable the policy. We think the policy can still stand. It may need some legislative adjustments."
At present, tests are done on day three and day 12 of the 14-day isolation period. It can take several days for an infected person to show up positive, so testing negative three days before a flight wouldn't remove the need for 14 days of isolation and two more tests once in New Zealand.
Dr Reti refused to say what experts the party had spoken to in developing the policy, saying National would like to "maintain those relationships".
"They'll identify themselves. We're grateful for those networks of trust and the relationships that we have, and if they wish to identify themselves as contributors to the policy, then that would be fine - but we want to maintain those relationships going forward.
"The policy will stand on its own merits. I think New Zealanders can look at it and say, 'We understand that.' We think it's a good idea to have point-of-departure testing before people hop on a plane, and we think on its merits, the policy stands on its merits. New Zealanders can tyre kick it."
The election is currently scheduled for October 17, pushed back from the original date of September 19 at the request of the National Party to allow more time for in-person campaigning, after alert level 3 restrictions were placed on Auckland earlier this month.