Does the bubble change our border risk profile?
Experts say the risk of "bubbling" with Australia remains extremely low.
That's because, like New Zealand, Australia has aggressively pursued an elimination strategy that's cleared the coronavirus out of the country.
As of today, Australia had reported zero new locally-acquired cases in the last 24 hours, and just five in the last seven days.
Per million of population, Australia's cumulative case rate since the start of the pandemic stood at 1151, compared with New Zealand's 523.
Both of those rates were tiny compared with other countries like the US (93,006 per million) and the UK (64,470 per million).
Australia and New Zealand also both continued to test at comparatively high rates, and scored generally high on the response-measuring Government Stringency Index.
Because there was no virus circulating in Australia, adding an extra 25 million people to our bubble essentially changed the risk profile little, Covid-19 modeller Professor Shaun Hendy said.
"Australia has a really good surveillance system, so we can have quite a lot of confidence," he said.
"When they say there's no transmission, there's no transmission. Having said that, if the situation changes in Australia, then the risk can go up."
Fellow Te Punaha Matatini modeller Professor Michael Plank said if there was a community outbreak in Australia, the risk of importing it into New Zealand could rapidly escalate at pre-Covid travel volumes.
"This is why it will be critical to act swiftly if this does happen."
Plank said the response to a new community case would be based on the same sort of decision making we have seen used here - so travellers should be prepared.
Auckland University infectious diseases expert Associate Professor Siouxsie Wiles agreed the risk of travellers being infected appeared low.
That was provided measures like keeping airline staff, passengers and processes separate in airports were managed properly.
While the bubble did add an extra layer of complexity, it was one many people seemed to want, she said.
"But then I've had emails from people saying we shouldn't be doing this until we're all vaccinated."
As both countries were committed to stopping community transmission, she said travellers, too, had to ensure they were doing all they could to minimise risk.
Commentators have suggested other virus-free jurisdictions like Taiwan could eventually also enter the bubble, but Wiles pointed out any expansion would now have to be decided jointly by the two countries.
Are Australia's borders looser?
Otago University epidemiologist Professor Nick Wilson argued the opposite: it was New Zealand that needed to beef up its border more.
A recent analysis by Wilson and colleagues suggested New Zealand faced a risk of as many as nearly three border breaches each month - and by some measures the country was more threatened than Australia.
They looked at nine border failures in New Zealand, including the August Auckland cluster that resulted in around 180 cases and three deaths, and seven in Australia - six of which led to lockdowns.
Using the estimated number of people who went through both countries' hotel-based quarantine facilities up to the end of January - along with the equivalent number of positive cases - they calculated a combined failure rate of one per 20,702 travellers.
That also worked out to one failure per 252 positive cases in quarantine and one outbreak leading to a lockdown per 47,319 travellers.
When broken down by country, New Zealand had 15.5 failures per 1000 positive cases going through quarantine, compared with two per 1000 in Australia - a seven-fold difference.
The researchers noted that, given the proportion of cases per 1000 had risen to 9.1 in New Zealand and 16.3 in Australia, that equated to 1.8 and 0.7 expected failures per month, respectively.
The researchers said New Zealand's comparably riskier rate could be down to either a "lower-quality" approach - or possibly better detection, from testing more border workers over a longer period.
Still, they added that, since December, the proportion of positive cases among international arrivals was still greater in Australia than in New Zealand.
"Australia is also accepting far fewer people from red zone countries on a per capita basis - the last time I looked, it was about five times less," Wilson said.
He has urged the Government to undertake a "bench-marking" exercise with Australia, to align border measures.
That included adopting Australia's MIQ requirement that no one leaves their room for the whole 14-day quarantine period - and Wilson has recommended New Zealand scrap shared spaces and exercise outings in its MIQ system.
Experts have also sounded concerns about freed-up MIQ spaces here being filled with more travellers from red-zone countries - thus raising the risk at the border.
The Government says up to 1300 MIQ spaces would be freed by the bubble - of which 500 would be kept as a contingency for the transtasman arrangement - but insists the changes won't raise the danger.
Will we still be able to trace outbreaks as easily?
While both countries also have excellent contact tracing systems, scanning isn't compulsory everywhere across the Tasman, just as it isn't here.
Wilson argued that using tracing apps should have been a mandatory measure for people in their first two weeks of travel in both countries.
But University of Auckland tech expert Dr Andrew Chen didn't think that was feasible here, given that giants Apple and Google would see such a move as a breach of their rules, and stop providing Bluetooth tracing.
"So I don't think the Government is going to go down that path."
Australia has a universal Bluetooth app, Covid Safe, but it's unclear how many people use it because the federal government doesn't release the data.
Chen, a research fellow at Koi Tū: The Centre for Informed Futures, said state-level apps seemed to have much higher uptake.
"That is mostly because they are mandatory in certain settings - hospitality, mainly," he said.
"So I would expect that if somebody was coming to New Zealand from Australia, they would be quite used to having to scan in.
"Hopefully, they would get instructions on how to turn on Bluetooth tracing when they arrive here and go from there - but if their experience in Australia hasn't been as good, they may choose not to do that."
For Kiwis heading to Australia, he recommended they research the tracing requirements of the state they were travelling to, and that their phone was compatible with the local app.
"It is particularly important for people travelling across borders to participate in digital contact tracing, as air travel allows people to travel and therefore potentially transport the virus large distances very quickly," he said.
"Combined with more transmissible variants of the virus, speed becomes more important in containing the spread of Covid-19."