More New Zealand wastewater plants will be screened for traces of coronavirus, in an expanded trial that could ultimately offer the country a new way to pick up early outbreaks.
Science agency ESR has already developed a methodology for detecting viral fragments of SARS-CoV-2 in wastewater, from initial sampling at Auckland's Jet Park Hotel quarantine facility.
That's now to be expanded to other centres with managed isolation and quarantine (MIQ) centres around New Zealand - East Auckland, Christchurch, Hamilton, Rotorua and Wellington - in an eight-week pilot study.
Sampling will also take place in suburbs and cities - including West Auckland and Rosedale, Dunedin, Gisborne, Invercargill, Napier, central and north Nelson, New Plymouth, Palmerston North, Queenstown, Taupō and Whangārei - with no MIQ facilities.
The sampling will allow researchers to prove their methods work, while also helping them evaluate how sensitive they are, and what improvements need to be made.
ESR's science leader, Dr Brent Gilpin, said the pilot was focused on how logistics of sampling could work - and that wastewater testing wasn't about replacing clinical testing, but a potential way to prompt or guide it.
"The project is aiming to establish how sewage sampling results may be combined with other established epidemiological surveillance data, like our current gold-standard testing regime of PCR testing of nasopharyngeal swabs," he said.
"The established tools will remain the most sensitive and rapid way of identifying an outbreak. We want to add value to that through this research."
If ESR did happen to find unexpected positives, there was a plan in place in which the result would be confirmed and reported to the Ministry of Health.
ESR environmental virologist Dr Joanne Hewitt said interpretation of negative and positive results can be a challenge.
"The context within which you interpret the data is just as important as how you detect it," she explained.
"For example, virus fragments can still be detected in the stools of some people for weeks following recovery from Covid-19, and these can be detected in the sewage.
"This means that a positive result does not always mean that there are active cases in the community."
Conversely, she added, a negative result did not necessarily imply that there are no active cases either.
"Not everyone with Covid-19 will excrete the virus in their stools, and the detection methods have limited sensitivity - we are still understanding these factors."
Once the eight-week period was over, ESR would review the data and make recommendations to the Government on any further wastewater sampling.
"The country would need to explore options to fund and organise any ongoing or expanded surveillance, but that is a wider decision to be made by the Government which they need good science to lead their decision."
ESR is part of an international collaboration, led by Water Research Australia, aimed at developing the approach to combat Covid-19.
New Zealand researchers have been calling on the Government to push harder on it, arguing our progress was slow compared to Australia, where detection of the virus in Sydney's sewage recently prompted an alert for dozens of suburbs.
Otago University epidemiologist Professor Nick Wilson was encouraged at the widened pilot.
"It will take some time until we really know how useful it is in the New Zealand context," he said.
"But this a good country to further evaluate this type of system. It's already got quite a long track record in terms of identifying polio viruses, along with illicit drugs."
Wilson and his colleagues have meanwhile suggested a raft of other improvements to New Zealand's testing regime to keep Covid-19 out - including more community testing, and more frequent testing of border workers.