Epidemiologists say there's no cause for alarm over two "weak" positive coronavirus cases picked up in Wellington wastewater, with similar traces from past infections having been found before through sewage sampling.
But they're nonetheless urging anyone in the capital, or who have recently visited, to get promptly tested if they have Covid-19 symptoms.
A weak positive result from a Wellington wastewater sample was reported late on Friday, before another was detected from a follow-up sample taken on Saturday.
"Based on our experience, it is most likely that the two weak positive results are due to recently recovered cases continuing to shed the virus," the Ministry of Health said in a statement.
In recent weeks, three recovered cases who live in the Wellington region have left the Auckland quarantine facility at the Jet Park hotel.
A further historical case left a Wellington managed isolation facility the same day the second wastewater sample was taken.
"Additionally, it is possible that one or more recently recovered cases from elsewhere could have flown into Wellington," the ministry added.
Last week, a ministry spokesperson told the Herald how SARS-CoV-2 RNA could also be shed for a number of weeks following a person's recovery.
"We have seen this with wastewater testing to date in New Zealand where virus RNA fragments were detected in wastewater samples collected at a location near the homes of people who had recently recovered from Covid-19."
Sampling of city wastewater plants for traces of the virus has been carried out in New Zealand since April last year.
In ongoing trials, ESR has been testing centres with MIQ facilities, along with other cities and suburbs without them, to look at how wastewater might be potentially used as an early warning system for outbreaks.
Capable of detecting coronavirus RNA shed in faeces by a single infectious person, the approach can also offer a way to indicate an increase or decrease in case numbers.
While ESR scientists have been able to sequence genomes from virus traces found in wastewater - something which could link them to known cases - that was difficult to do when there was little genetic material to draw from.
Otago University epidemiologist Professor Nick Wilson agreed that past infections appeared to be the likeliest explanation for the new cases.
"It would be great if they could do some genomic work on these fragments, but it doesn't look like they can," he said.
"But it's also a good opportunity to remind people to get tested if they've got symptoms, and they're in the Wellington region, particularly."
Fellow Otago epidemiologist Professor Michael Baker said wastewater testing appeared to be a useful system in a country like New Zealand that had effectively eliminated the virus.
"But we need to know how effective it is in picking up small outbreaks, or whether we're more likely to keep picking up false positives."
The new detections come after Wilson and colleagues last week suggested that New Zealand could follow United Arab Emirates, and look at testing airline sewage in the future as a way to screen infection rates from red zone countries.
A positive test from aircraft sewage on an incoming flight could prompt the need for "heightened awareness" among aircrew for symptoms, Wilson said, as well as full compliance with daily saliva testing.
For passengers transiting into managed isolation and quarantine (MIQ), it could prove another tool to detect cases early, and move affected travellers to appropriate facilities.
Currently, the ministry said this wouldn't add any value, given our border regime already required PCR tests from travellers on red zone flights on their first and third days here.
ESR science leader Dr Brent Gilpin said the institute had also looked into testing of wastewater from flights.
"Aside from the obvious - not everyone deposits faeces on a flight, if it were to be employed, there are quite a few operational issues to overcome including cleaning procedures of the tanks between flights, sampling, speed of analysis, response and other issues like shedding in the post-infectious phase."