National says there should be a much better system to collect money for stays in managed isolation and quarantine (MIQ), including using debt collectors when bills are overdue.
The government is waiting on 956 people to pay overdue MIQ fees.
To date $11.7 million has been paid but there's a total of $20m outstanding.
National MP Chris Bishop said New Zealanders would expect people to be contributing to the total cost, and the system should be better.
"They should really be invoicing people properly, but also chasing people down and we should be using debt collection agencies, for people who are really late in paying the bills."
But the government also needed to make it easier for people to pay, he said.
"Take credit card details up front when people book the spots. Most people will be familiar with checking into a hotel, you put your credit card down, and we should be doing the same thing with MIQs, after all, they actually are literally hotels."
Returnees who don't meet fee waiver criteria have 90 days after leaving the hotel to pay for their stay.
But there's $3.2m owing from people who haven't met that deadline.
The Ministry of Business, Innovation and Employment says another $17m is owed by people who are still within the 90 days.
No late-payers have been referred to debt collection.
Prime Minister Jacinda Ardern said it was a privilege to go through MIQ and people who haven't paid yet, should do so.
"I think people should pay. Ultimately we have been asking New Zealanders to limit their movements because it's unsafe. For those who are taking up the privilege to be able to leave and then return safely, we ask them to do the right thing and pay."
The government is unable to stop people leaving the country if they haven't paid. But Ardern said the government could collect overdue payments at the border when people came back into the country, as it does with student loans.
Covid-19 Response Minister Chris Hipkins said the system needed to be better and he was getting advice about whether overdue payments could be collected at the border.
At the moment that's only possible when people are coming into New Zealand.
"I'm looking at how we can tighten some of that up, I'm not happy about the fact there's this amount of money outstanding," Hipkins said.
"They may not be in a position to pay upfront, it is handled separately from the hotels so we pay the hotels and then we invoice the people who stay there but certainly the invoices should be issued very quickly, more or less immediately after someone has completed their stay at MIQ".
"It may be that we need some primary legislation in order to be able to do that so a law change through Parliament... at this point I haven't got evidence to suggest it's a widespread problem but even a few people leaving would be something that I would regard as pretty unacceptable," Hipkins said.
Bishop didn't believe the government should go as far as stopping people at the border if they had an outstanding bill.
"It's hard to stop people literally leaving the country... but what we can do is make it easier for people to pay and chase them when they don't."
He said if "push came to shove", collection agencies could be used overseas.
"If people have skipped the country, we can get that money back off them. There are ways for the government to do that and they should be exploring them and using them."
Sebastian Clarke, who came to New Zealand from New York to visit his family in January, told Midday Report that he was still waiting for his bill.
Clarke told RNZ he had yet to receive $3100 bill for his hotel isolation stay, but was wanting to pay it.
An MIQ spokesperson said Clarke was emailed an invoice for his stay on 4 February.
The spokesperson said there may be an administration error, and urged Clarke to get in touch.