One of the reasons a trans-Tasman bubble won't mean more space in MIQ is because potentially hundreds of people would need to be isolated if there was an outbreak on a flight.
COVID-19 Response Minister Chris Hipkins gave the explanation after saying on Thursday he couldn't guarantee a trans-Tasman bubble would increase managed isolation availability despite travellers from Australia making up about 40 percent of spaces.
"We have to keep some space aside for contingency because ultimately, when we're having thousands of people every day flowing across the Tasman, we have to allow for the fact that if there was a COVID-19 positive person on one of those flights for example, then we would need to isolate that flight," Hipkins said.
"We'd need to make sure that we've got capacity to be able to do that."
Australian passengers make up about 1800 of the 4500 spaces in MIQ. But the average capacity on flights between Australia and New Zealand is only about 200 passengers, so it couldn't be the Government's only concern.
Hipkins said the Government also needs to weigh-up the higher concentration of risk that would come with more arrivals from countries where COVID-19 is still rampant. France and Italy, for example, are going back into lockdown.
"The second thing is, as we open up more space for people coming from further afield, we know that we'll get more positive cases, and so we just have to think about how we manage that within the facilities," Hipkins said.
"[It's] making sure that we're spacing the facilities out so that we're not creating more risk by having a higher concentration of positive COVID-19 cases."
Evidence of pre-departure tests from passengers, as well as day one testing in MIQ, has helped to reduce the risk, Hipkins said. But the higher the risk of the passenger cohort, the higher the risk overall, he explained.
Hipkins said it has nothing to do with the quarantine hotels ending their contracts.
"No not at all. If there is any reduction in hotels, it's likely to be us making the decision on the basis that the facility might not be suitable," he said.
"By and large, there has been a good degree of enthusiasm from the hotel operators to continue to offer those hotels to us."
Newshub revealed the Government could increase MIQ by about an extra 3000 rooms, making space to reunite families and bring home stranded Kiwis, but it's choosing not to open them.
The Ministry for Business, Innovation and Employment (MBIE), which is responsible for MIQ, says 55 facilities were originally deemed suitable. Just 32 are currently operating so that means 23 are available.
Epidemiologist Michael Baker has warned that filling vacant spots after a trans-Tasman bubble is in place with people from COVID-19 hotspots is dangerous and that it would almost double the risk of a community outbreak.
But he says Australia's risk profile is low enough that the bubble can happen.
Where's the bubble?
Hipkins said Cabinet is yet to make a final decision.
"I don't want to put a particular timeframe on it. It'll take as long as it takes. We're working at pace, we want to get it up and running as quickly as we can, but we want to make sure people understand what the risks are," he said.
"We've shifted our position from trying to negotiate a joint decision-making framework with Australia to something that's more unilateral, which gives New Zealand and Australia the ability to make their own decisions about when, for example, to suspend safe travel between the two countries.
"That's going to create a degree of uncertainty for people who are travelling so we just need to understand that uncertainty so that people who are travelling between New Zealand and Australia know what to expect and they can plan for that."
A travel bubble would open up quarantine-free travel within New Zealand and Australia, but each country would retain the right to halt such travel as it sees fit.
Airports would be divided into 'red zones' - for travellers from countries where COVID-19 is prevalent - and 'green zones' where COVID-19 is more or less under control. Transit passengers would have to catch separate flights.
It's understood Australia cooled on the idea of having a joint set of rules, which is what New Zealand had been working towards. Officials had concluded 11 rounds of talks with Australia on this proposed model.
Things got complicated when Australian states started opening up quarantine-free travel New Zealanders on their own, rather than nationwide.
Then in January, Australia suddenly shut off quarantine-free travel for New Zealanders in the wake of the Northland community case, which angered Prime Minister Jacinda Ardern at the time because it left Kiwis stranded.
The following month it's understood Australia became more favourable towards that approach - that each country should have the right to suspend travel as they see fit.