A Covid-19 modeller is forecasting 200 community cases a day of the coronavirus by Wednesday and 400 daily cases by the end of the week.

Te Pūnaha Matatini modeller Dr Dion O'Neale says because Omicron is in the mix, this week would mark a shift from the previous relatively low case numbers.

"Almost every other place in the world that's had an Omicron outbreak's seen a doubling time of around three days. We'd expect New Zealand to be similar.

"So that means about 100 cases a day at the start of the week, around the middle of the week we're probably looking at around 200 cases a day, and then doubling to around 400-ish by the end of the week.

"Maybe things will go badly - that will arrive on the Thursday or Friday, maybe things will go well and we'll make it to Sunday before we get to that level - it's that rough pattern of taking around three days to double."

Dion told RNZ's Morning Report that even though not all current cases are being confirmed as the Omicron variant, modellers expected it would be the majority soon and were now making their calculations based on its behaviour.

"Omicron cases grow faster than Delta and other variants, so we expect it to out-peak other variants and take over... it's safest for us to be assuming that a case that comes up is Omicron unless we know otherwise."

He warns Omicron has a very fast incubation period, or latent period - the time between a person getting infected and the point they showed symptoms and became infectious themselves.

Once a person realises they are sick, takes action to get tested and their results are reported, it could be four or five days from the point of infection.

"That's important to remember: the case numbers we're seeing at the moment, those are infections that are already baked in, that happened some time ago."

This also means that patterns showing comparatively fewer people getting tested for Covid-19 in weekends or on public holidays skew the numbers: "That will affect how those case numbers grow in the short term," he says.

RNZ.