Covid-19 mainly came to New Zealand from the United Kingdom and the US, but also possibly via places where there are no confirmed Covid cases, including Antarctica, the Cook Islands, and Vanuatu.
There have been 588 cases brought into New Zealand from overseas, and country of origin data has been provided to the Weekend Herald from the Institute of Environmental Science and Research.
As far as possible, the institute has tried to pinpoint the first country of departure from where Covid-infected travellers started their journey back to New Zealand.
Most prominent were countries where Kiwis like to travel to, including the UK (188 - combining the tallies for the UK, Scotland and England), USA (123), Australia (40), Canada (23), France (19) and Ireland (14).
The data does not include the nationality of the people, but after March 20, when the borders were only open to Kiwis, most returnees flew home from one of these countries.
• Covid 19 coronavirus: New Zealand has three new Covid-19 cases today - one in Rotorua and two in Christchurch
• Covid 19 coronavirus: Revealed - more than 1000 people may have left isolation without a test
• Covid 19 coronavirus: Health chief Ashley Bloomfield reveals two new cases of Covid-19, one in locked-down Rotorua hotel
• Covid 19 coronavirus: Ministry of Health raises criteria for testing
The data paints a general picture of how Covid came to New Zealand, but it should be interpreted with caution.
The country of departure may not be where the case was acquired, ESR public health physician Sarah Jefferies said.
"Cases may have been travelling in several countries on the journey back to New Zealand within the incubation period of Covid-19; cases may have been exposed during transit in another country; and cases may have been exposed on a flight."
One case travelled from Antarctica, one from the Cook Islands, and two from Vanuatu - areas where no confirmed cases have been recorded. These people may have carried Covid undetected through these regions, or become infected after they had flown out.
Otago University epidemiologist Professor Michael Baker said there had been a relatively low number of cases from Australia in light of the high volume of passengers.
"The encouraging thing is that most of Australia is now on the successful elimination path like New Zealand, and despite reasonable numbers coming from Australia, very few recently are positive."
That could bode well for a transtasman travel bubble, he said.
"I don't think people should be too pessimistic about the potential to link up with Australia."
Auckland University Professor Shaun Hendy, who has provided modelling to the Government on the spread of Covid, said the data reflected how Covid had spread around the world.
"We can see a shift from cases predominantly coming from the UK and Europe to a more widespread world-wide distribution later on. This reflects how the disease has spread globally as well as changing travel patterns."
He said there were about four cases per 1000 international arrivals, and it wasn't a surprise to see the UK being the largest source of infections.
"This probably partly reflects the large numbers of Kiwis there who headed home, but also how big the first wave of the epidemic got there.
"This is consistent with what we know from the genetic analysis of the viral infections in patients here - that most of our Covid-19 cases were linked to Europe."
Baker said travellers tended to be younger and it would be interesting to know their demographics. He said 20 per cent of Covid-infected people aged under 20 showed symptoms.
Testing of overseas arrivals in managed isolation facilities was now mandatory, and Baker said he expected more positive tests.
"Before they were routinely tested, if they didn't have symptoms they went into managed isolation for two weeks and then were released. That's fine and very robust.
"But now everyone is being tested, and we have greatly increased the number of positive cases being detected. If you start to get flights coming back from some of the high transmission zones, there might be a rush of cases."
But that wouldn't necessarily affect the elimination status of Covid-19 in New Zealand, he said.
He and other public health experts have defined elimination as the absence of any local transmission for a few weeks, high testing rates and the effective quarantine of any imported cases.
The recent cases may seem like an outbreak, but the last case of community transmission was on April 30, and the last locally transmitted case was over a month ago.
Most of the imported cases started making their way to New Zealand in mid-March, around the time Prime Minister Jacinda Ardern asked almost all overseas arrivals to self-isolate for 14 days.
When that failed as an effective border closure, Ardern closed the border to all non-Kiwis after finding sub-par procedures at the border and amid reports of people flouting isolation expectations.
The Ministry of Health recommended closing the border to all people, including Kiwis, but it was rejected by Cabinet because it is an inalienable right of New Zealanders to be able to come home.
Researchers at Te Punaha Matatini say around 12 new cases could be expected each week in New Zealand, although this figure could vary depending on arrival numbers.
That was partly down to the rising volume of travellers – climbing from 1000 per week in mid-May to more than 2500 in the past week – but also the acceleration of the pandemic around the world.