When (or perhaps if, depending on the Melbourne outbreak) Australia's Prime Minister Scott Morrison arrives in Queenstown on Sunday, there will be more than political differences between he and Prime Minister Jacinda Ardern.
Morrison is vaccinated against Covid-19. Ardern is not.
The danger Ardern faces is that state of affairs comes to be seen as reflecting the nations each leader represents. Australia gets vaccinated, New Zealand waits.
The two leaders will meet as the transtasman bubble faces what is shaping up to be its biggest test yet: the outbreak in Melbourne.
Some might have expected Morrison to cancel his trip to New Zealand during an outbreak in his homeland. But for both leaders there is a lot at stake.
There was some political risk in opening the bubble at all, as nervous people wondered if it was too risky.
Both leaders will be at pains to show that they can handle and contain outbreaks in one state of Australia or in New Zealand, without putting either the elimination goal or the wider bubble at long-term risk.
Prior to Melbourne, that had been the case.
The borders were slammed closed close for a few days in earlier outbreaks, but quickly re-opened.
But the Melbourne outbreak is something bigger.
Thus far, it has been dealt with by New Zealand in exactly the way the public were told to expect.
The "flyer beware" condition Ardern put on the bubble at the start was invoked, and people who travelled to Melbourne were told they now have to stay there and abide by the Melbourne lockdown rules for at least the next week.
The bigger challenge is dealing with those people who were in Melbourne during the infectious periods of the original cases, and who have since returned to New Zealand.
The Ministry of Health has pointed to the difficulties in this– there are now a large number of locations of interest, and they are still growing as more cases are confirmed in Australia.
That is where the most dangerous part of the bubble's rules come into play: the trust-based part.
The Government is trusting Australians visiting New Zealand, or New Zealanders who had just returned, to be truthful about whether they were at one of the locations of interest. For some, that is no small ask – it could result in that person being told to isolate for two weeks.
It would be easy to lie. It would also be easy for someone to simply forget they were at one of those locations, especially if they were not tracking their movements.
If the Melbourne outbreak is not quickly contained or it spreads elsewhere it will have a longer-term cost.
There will be a chilling effect on transtasman travel as people weigh up whether they can afford the time, or the cost, of that "flyer beware" condition. Trust in the safety of the bubble will also be undermined.
And it will certainly make the Government – and New Zealanders – more wary of bubbles with other countries.
The one thing that would put bubbles on firmer ground is vaccinations.
It is the inescapable fate of New Zealand governments to be constantly compared with Australia – over everything from wage gaps, cost of living and house prices.
The year, the comparison is firmly focused on the vaccinations rollouts.
According to Oxford's Our World in Data tracker, about 13 per cent of Australians have now had at least one shot. In New Zealand, that is only 7.7 per cent.
Despite that, the Australian Government is constantly under fire over the pace of its rollout. Other countries are much further ahead – Canada and the United Kingdom have now vaccinated more than half their populations.
The drubbing Morrison got after delays to Australia's planned rollout perhaps politically justified the New Zealand government's reluctance to issue firm targets.
But New Zealand's people have so been relatively patient. Most people seem to have accepted the Government's justifications for being behind and are content to wait.
But Covid Response Minister Chris Hipkins has warned the rollout could need to be slowed or even paused because of vaccine supplies. The rollout to the 2 million adults in the general population has now been pushed back to the end of July.
If that rollout starts to move into next year, the patience may wear thin.
Australia's rollout has been helped by using more than one vaccine, such as AstraZeneca, so it is not as reliant on Pfizer's supply.
That is New Zealand's plan B – Hipkins has said it could call in other vaccines, such as AstraZeneca, which is going through MedSafe approval processes. The biggest risk there is that the Government has sold Pfizer as the gold standard – and people might be reluctant to get another variety.
As New Zealand's rollout increases, there are other lessons from the Melbourne outbreak. It highlighted that it was not vaccine hesitancy that was an issue: but complacency.
The initial cases in the Melbourne outbreak were all eligible for vaccines, but none had got them.
There was a surge in people getting vaccinated in Victoria after the outbreak.
Outbreaks don't only break complacency among the population: but governments as well. Victoria moved immediately to open up vaccinations to everybody over 40.
In a Covid-free land, it is easy to put off a vaccination for another day.