A third wave of the Omicron variant of Covid-19 is likely to hit the country this year, Professor Michael Baker says.

Covid-19 cases are on the rise again with over 16,000 cases reported over the past week - an increase of about 2000 cases in a week.

Waitematā, Auckland and Counties Manukau have reported almost 6000 cases in the city alone.

As well, as of midnight on Monday, there were 243 people in hospital with the virus, with six in intensive care.

Professor Baker, who is an epidemiologist at Otago University, says there has been two big waves of Omicron this year and there looks like there will be a third one, but it was unknown at this stage how big it will be.

He says Singapore, whose experience of the pandemic has been similar to New Zealand's, is also experiencing a steady rise.

"That may be what we'll see here."

Two new names have become prominent - the BQ.1.1 is a sub-variant while XBB was a recombinant variant which had grabbed genetic material from two other sub-variants. The latter is driving Singapore's surge, Baker says.

Both are on the country's watchlist and have been detected in some cases and in wastewater in New Zealand.

While the country is heading into summer, the time of year doesn't affect the sub-variants because they are so infectious, he says.

"So they get a bit of a boost from winter when people are indoors more but they don't absolutely need winter conditions to spread."

He says it's "slightly worrying" to see cruise ships back in New Zealand waters with about 130 infected people on one ship that has berthed at Wellington.

However, with open borders and 10,000 people flying in daily from overseas, cases on cruise ships will be a small contribution to the country's Covid-19 case tally.

Baker is concerned that many adults have not lined up for their booster shots.

"So that's really a critical step and that's still the best measure that people can take to protect themselves and it does help against these new variants as well."

-RNZ.