To build a snapshot of undetected spread around the country, the scientists searched for specific antibodies among some 9806 samples collected from New Zealand Blood Service donors aged between 16 and 88.
Antibodies play a critical role in the immune system's fight against pathogens like the coronavirus.
Upon a new virus being recognised, antibodies are specially created to bind to its "spike protein" and stop it entering our cells - all while signalling other parts of the immune system to destroy the foreign invader.
Antibodies could also act as handy markers of past infection.
It was antibody sampling that allowed scientists to last month reveal a previously-unknown cluster that pre-dated New Zealand's first officially reported Covid-19 case.
A co-author on the new study, University of Auckland PhD student Alana Whitcombe, said determining the number of samples positive for Sars-Cov-2 antibodies could essentially help gauge the level of undetected transmission and potential spread of the virus across the country.
That was crucial, given there was evidence to show that up to 15 per cent of people contracting Covid-19 could have no symptoms, but still be infectious.
When it came to sampling, Whitcombe and her colleagues measured the antibodies using tests based on the viral spike protein.
"That's important as the survey samples were collected at the end of 2020 - yet the first case of Covid-19 was reported in New Zealand in February 2020," Whitcombe said.
"Basing our survey on long-lived spike antibodies meant we were most likely to detect all possible past infections, even those that occurred in early 2020."
Of the total samples, the researchers identified 18 positive samples.
To ensure they could be certain of their results, the team also re-confirmed positive samples using two further commercially produced spike-based assays.
Six of those 18 were confirmed as having previously had Sars-CoV-2 infections and a further four came from people who had travelled from the UK and Europe, suggesting infection outside New Zealand.
The remaining eight "seropositive" samples came from seven different district health boards - equating to a total seroprevalence rate of 0.1 per cent of the population.
Study co-author and University of Auckland research fellow Dr Reuben McGregor said this was only a little higher than an earlier reported prevalence of about 0.04 per cent.
While their results suggested some undiagnosed infections have occurred, McGregor said it was far lower than seroprevalence estimates of more than 10 per cent, reported in studies overseas where the virus had been poorly controlled.
"Overall, the survey shows that undetected community transmission appears to have been limited in New Zealand and it provides serological evidence for our countries elimination success," added study author Associate Professor Nikki Moreland, an immunologist and biomedical scientist at the University of Auckland.
The data also provided a good baseline for the vaccine roll-out, she said.
"While the seroprevalence detected in the survey is very low there is no reason to think that our levels of immunity might be affected by a lack of circulating virus."
Clinical trials and real-world studies overseas had shown that two doses of the Pfizer vaccine produced levels of antibodies and immune responses that were in line with - or often superior to - those seen after a Covid-19 infection.
"Achieving high vaccine coverage and increasing seropositivity for spike antibodies via vaccination is much safer than having Sars-CoV-2 circulating uncontrolled in the community."
The researchers noted that study came with some limitations - notably that it wasn't representative of all Kiwis, but just those eligible to be donors, and that it wasn't always reliable to extrapolate from such a small number of seropositive cases.
Otago University clinical microbiologist and infectious disease expert Professor David Murdoch also stressed that blood donor populations typically differed from the total population, in terms of age and ethnicity.
"However, the study findings do provide evidence that undetected community transmission of Covid-19 has been limited in New Zealand, and is consistent with similar findings from Australia," he said.
"In contrast, the prevalence was much lower than in regions of North America and Europe where the pandemic has been poorly controlled."
Otago University immunologist Associate Professor James Ussher said the findings also highlighted the effectiveness of our bold measures to eliminate the virus.
"This study confirms that lockdown and border restrictions were highly successful in limiting SARS-CoV-2 infection in New Zealand."
