New Zealand's COVID-19 outbreak continues to lessen, with 90 new community cases reported, although concern remains as new areas are threatened.

Wednesday's daily case count is the smallest in more than six weeks, with experts attributing the reduction in infection numbers to the growing vaccination rate.

Some 88 per cent of New Zealanders aged 12 and over are fully vaccinated, with 94 per cent having received a single dose.

Cases are down from daily counts above 200 recorded several times in November, though wastewater testing leaves cause for concern.

Health officials believe the virus could be spreading in the Tairawhiti region, one of the least-vaccinated parts of New Zealand, based off wastewater testing in the east coast city of Gisborne.

A health ministry spokesman said three positive results in the last week "strongly indicates that there is at least one undetected case in the community".

"It's a bit of a mystery," COVID-19 Minister Chris Hipkins said.

"I'd like to see an increase of the testing of people within the community so that we can identify more about what's going on there.

"They're doing more targeted wastewater testing to try and ringfence exactly which part of the Gisborne community the positive wastewater testing is originating from."

The Tairawhiti region has the second-lowest vaccination rates in NZ, behind Northland, with Maori immunisation rates lower still.

Of Wednesday's 90 new cases, all but one - in Nelson - was found on North Island, including 71 in Auckland.

COVID-19 modellers have tipped the virus to spread further around New Zealand from next week, when the government dismantles the border around Auckland and allows travel in and out of the COVID-19 hotspot.