The two weak positive results are most likely due to recently recovered cases continuing to shed the virus, the Ministry of Health said yesterday.
"This is evidence that there are people in Wellington who are shedding fragments of the virus and they're being picked up by the system," Baker said.
"In some ways it's a good finding because it's showing that the system can detect fragments of the virus when they're being passed out into the sewage system."
Australia was doing more extensive sewage testing for Covid-19 than New Zealand and it was time to compare notes on how sensitive the system is.
"The system is good at picking up actively infectious cases in a particular area but we really don't know how large that number would need to be to trigger a positive result," Baker said.
Wastewater around the Jet Park quarantine facility, for example, was routinely positive.
Baker said it was a good time to look at whether this was a system New Zealand needed over the next few months when the country was still vulnerable to outbreaks, and whether more sampling should be done.
"It's good to have more than one surveillance system, obviously.
"At the moment the role of sewage testing is still a bit unclear in terms of whether it is going to give us a big edge, or whether it will mainly pick up outbreaks that are already apparent through other means.
"I do think we're probably at the stage of hopefully doing a major review of this system, and I guess continually reviewing of all of our surveillance systems to see what's the best mix of approaches."
An example would be whether more testing should occur around South Auckland, where many border workers and managed isolation staff live.
"The role of this still needs more discussion."
ESR said in February it had been conducting routine sampling for many months from Auckland Western and Eastern Interceptors, North Shore (Rosedale), Hamilton, Rotorua, Wellington and Christchurch.
During the February cluster it carried out extra sampling from Auckland, Hamilton and New Plymouth.
