Wastewater testing has indicated there were far more Covid-19 cases out there than showing in the official numbers, the Ministry of Health reports.
Institute of Environmental Science and Research molecular biologist Dr Brent Gilpin told Morning Report only people with symptoms were likely to test and of those that tested positive, only some would report their results.
"There certainly are more cases than what are being reported but it's really about understanding the trends and changes in those cases that we see."
Since mid-September there had been a steady increase of cases and while new strains were coming through, Omicron was still the dominant strain, he said.
"Variant soup" was one of the new phrases being used to describe what's happening with Covid-19 in this country at the moment.
The institute's head of bioinformatics & genomics Joep de Ligt told First Up some of that variant soup had began to arrive in New Zealand.
"It's still pretty manageable [in New Zealand] and the variant soup is what's described in Europe."
There was no longer just one variant, like Alpha, Beta, Omicron circling, but a whole collection of variants which may not get given a Greek letter name, he said.
The Omicron subvariant BQ.1.1 was on the rise in New Zealand and likely driving up cases, Ligt said.
Most variants now were descendants of Omicron, he said.
A 'variant soup' made them harder to predict, he said.
"The number of people who get any one variant is less, so the power to detect those characteristics [like severity], is also less."
Seven big variants which had caused waves internationally were being tracked here, he said.
"There was one report from Denmark where in a month they saw 150 different variants."
A large rise in cases that was persistent was a big concern, he said.
The biggest problem was not the variant New Zealand got, unless there was a particularly bad one, but a wave that would impact vulnerable people that were previously safe, he said.
Ligt hoped that we could reach a point where there was collective thinking that mask use would be helpful no matter the variant.
"The only safe prediction is that there is going to be a wave, how bad it is is going to be partially dependent on people's behaviour but also these new variants - and they are around and they are causing waves - and we will lose loved ones."