A Covid-19 community outbreak can't be ruled out from the New Plymouth wastewater results but it's very unlikely, Otago University epidemiologist Michael Baker says.

Last night the Ministry of Health announced officials were investigating after two positive Covid-19 test results were detected in New Plymouth wastewater.

This morning, Baker said this was good news because it showed the wastewater testing was working.

"That's why wastewater testing is done. If there is an undetected outbreak there ... it's an additional back-up measure."

The epidemiologist said the results were most likely from recovered cases continuing to shed the virus, which was going into the sewer.

"People could be positive for months after they recovered. You can detect RNA for six months after and that means they are not infected with the virus, but were still shedding fragments."

But he said it was possible someone who was back from quarantine-free travel from Australia was living in New Plymouth and was infected, and was potentially infecting other people.

Another possibility could have been shipping crews who transferred from Auckland to New Plymouth and had infected staff there.

"They are retesting those staff that previously had tested negative ... it is a possibility but it's very unlikely. "

It could be someone who lived in New Plymouth or someone who was just passing through.

The suggestion that the positive wastewater result could have come from mariner crews, which had recently passed Port Taranaki, was dismissed by the Ministry of Health as the ships never pumped their sewage into local sewage treatment plants.

Baker said the take-home message was if anyone living in New Plymouth had any symptoms at all, they needed to be extra vigilant.

"If you are living in the area and are experiencing symptoms then get tested," he said.

Meanwhile, health officials are investigating whether any recovered cases who live in the New Plymouth area had recently left a managed isolation facility.

"Additional testing is being carried out for a small number of port workers and nurses, who have been in possible contact with the mariners. The nurses have previously been tested for Covid-19 and returned negative test results," the ministry said.

There are no managed isolation facilities in New Plymouth.

There were 19 new cases of Covid-19 reported in MIQ yesterday and none in the community. The 19 new cases are all mariners in isolation.

On Thursday night, genome sequencing of the crew on the Mattina confirmed they had the Delta variant.

In total there are now 15 positive Covid cases on that ship. All crew who test positive will remain quarantined on-board.

The crew members, including the captain, have tested negative and were safely transported to on-shore facilities.