The Nurses Society warns there's not enough resources to cope if managed isolation and quarantine (MIQ) guests arriving from Australia are replaced by people from more Covid-19-infected countries, when a travel bubble begins.
There are renewed calls for a 'traffic light' system, allowing people into the country based on risk assessments.
A travel bubble with Australia will alleviate demand for MIQ by an estimated 40 percent, freeing some 1800 rooms, and the government is now pondering if people from other countries should be able to book them.
The start date for a trans-Tasman travel bubble will be revealed on 6 April.
Covid-19 Response Minister Chris Hipkins said some MIQ rooms were likely to sit empty as contingency, in case there was a small outbreak that meant some trans-Tasman travellers needed to go into isolation.
If epidemiologist Michael Baker has his way, the rest wouldn't be filled with more travellers from high-risk countries.
The people who spend the longest amount of time in the facilities agree. Nurses Society director David Wills said healthcare staff have already been grappling with enough risk and a "very heavy workload".
"If there is a travel bubble with Australia, meaning some space is freed up ... that should not mean, a one for one replacement, with arrivals from other countries. That would cause a net increase in workload - a problematic increase because that would mean more high-risk arrivals and more positive cases," he said.
"You cannot simply recruit staff in the numbers that would be necessary."
Wills said it was not an issue of rooms or space in facilities but a question of staffing resources.
He said healthcare workers would need to do more testing of returnees.
"Then a higher number of them are likely to be positive - in fact that's without a doubt. And that would put extra pressure on the quarantine facilities and all the other processes that are associated with that," he said.
Prof Baker would like any MIQ rooms freed up by trans-Tasman travellers to be removed from the system entirely.
However, the idea of cutting MIQ capacity has been rubbished by immigration lawyer Aaron Martin who said the existing system had not been meeting demand from returnees and critical workers.
Martin said he was amazed the government was not already scaling up the MIQ system.
"It's like there's now two borders, effectively. You can get your visa but you can't just get on a flight and get into MIQ. You've now got a several month wait to try and get a place in the MIQ system because it's so backlogged," he said.
"Trying to reduce the capacity would just seem to exacerbate that problem."
Economist and Covid-19 modeller Rodney Jones said there could be a compromise which limited the risk without reducing the overall number of returnees.
Of the 85 positive Covid-19 cases reported by the Ministry of Health in MIQ facilities this month, 50 were people travelling from India, six had arrived from the UK, five were from Ethiopia and five from Pakistan.
Countries that were previously a common origin of New Zealand's border cases, like the United States, are appearing less frequently.
Jones said the situation around the globe was changing and New Zealand could adapt.
"What we need to do on a forward-looking basis is assess risk. This is what a lot of public health commentators have been calling for - the traffic light system. Right now you would say the UK and the US are green and India, unfortunately, you'd say is red right now. You could make places available on that basis, you know 20 percent to green, 5 percent to red," he said.
Jones said whether the government was open to that idea would depend on its ability to adjust technology such as the MIQ booking system.
He said there was no knowing when the global rollout of vaccines would result in less cases at the border and warned it was "a long journey ahead."
Jones said New Zealand's focus should be on opening up to the countries it could while maintaining a cautious stance.
In a statement, the Ministry of Business, Innovation and Employment said it was currently considering the impact of a greater number of returnees from higher-risk countries potentially entering managed isolation facilities.