Cabinet Minister Megan Woods, who has ministerial oversight of quarantine and managed isolation facilities, said the legal advice was not to put an "economic impediment" for Kiwis returning home.
"That's what we're working through ... and it could well require legislative change."
"One of the things we also need to make sure is that we're essentially not setting up a test for New Zealanders based on how much money they've got in their bank account.
"We'd have to make sure we had hardship measures in there as well ... that people can pay it back over a period of time, for example."
Ardern said holiday-makers should face a full payment, rather than a co-payment.
"I'd say if you 're making the choice at your expense to travel overseas, then you should meet the full cost of that holiday."
Figures released to RNZ showed 60 people left the country after alert level 4 was introduced and returned before May.
To the end of June, the Government has estimated it would spend $81 million on moving 21,500 Kiwis through border facilities - at an average of $3800 per person.
"We wouldn't be making it illegal to return," Ardern said.
"It's whether or not we can impose a cost to make that happen. This is the question we're asking - is there a difference between a New Zealander returning versus and New Zealander choosing to leave and then returning?"
That was the same question the Government was seeking an answer for as it considers co-payments for Kiwis in quarantine or managed isolation, which is being implemented in Queensland.
With more borders opening up, National Party leader Todd Muller said the Government needed to tell the public the criteria for easing quarantine restrictions at the border in the medium and long-term.
"The idea that we stay locked up at the bottom of the world waiting for a vaccine without any forward view over what the strategy should be as other countries make changes ... that to me remains an untenable position.
"What are the rules? What needs to happen in their country in terms of tests and being proven to be Covid-free, and what changes might [there] need to be in terms of how we do quarantine and managed isolation?"
All overseas arrivals should be placed in quarantine or managed isolation in the short-term because of the recent border failures, he said.
"But we can't have a situation where in many months' time you fly to Europe, you have an enjoyable holiday there you can demonstrate that you're Covid-free but still when you come back to New Zealand you need to be quarantined for two weeks and have two more tests."
Asked whether mandatory quarantine was reasonable given the prevalence of Covid in Europe and the 14-day incubation period, he said people might not have to be in isolation if they could show they were Covid-free.
"I'm not the expert. I'm not saying it should be one day and no tests."
He was simply challenging "the idea that what we have now is locked in for perpetuity for weeks and months, well into next year, without any flexibility, without any sense to evolve depending on the risk".
Ardern said the criteria for opening borders with countries without a need for quarantine was similar to the transtasman bubble.
It included, for example, no community transmission in those countries and "being certain that people coning in who won't be required to isolate won't bring the virus with them".
The Prime Minister's chief science adviser Professor Juliet Gerrard and research analyst Rachel Chiaroni-Clarke have said several countries had contained the virus well enough for international travel to be considered - with appropriate risk-management.
Factors that need to be considered include the prevalence of active cases, the integrity of testing data, the level of community transmission, and each country's control measures.
Ardern said opening the economy up more was an important consideration.
"But we will not sacrifice the hard work of New Zealanders while we do that."
