Prime Minister Jacinda Ardern says a change in Covid-19 traffic light settings would not make a significant difference on surging case numbers.

She says the most important thing is for people to adhere to the current requirements - which means wearing a mask and those over 50 getting a second booster.

Covid-19 cases continue to increase with 9000 cases reported yesterday and hospitalisations rising to 662, the highest mark since 5 April.

Ardern has seen many members of her own Cabinet fall ill with Covid-19 in recent weeks.

She told Morning Report that existing measures are being constantly reviewed and will be discussed again on a weekly Covid-19 call.

She acknowledges it's a very challenging time for the health system, including for hospitals, however, the basic measures are the country's most effective weapons in dealing with the virus.

"Mask use and vaccination continue to be the most effective things we can be doing ... they reman critical so it's a role we can all play to keep assisting with what is at the moment a not unexpected surge but a difficult one of B4 and B5 [Omicron sub-variants] in New Zealand."

Covid-19 case numbers are now similar to early April when the country was at a red setting.

Asked why the government was not therefore returning to the red setting, Ardern says one reason is that up to half of New Zealanders have had Covid-19.

The other reason is the highly transmissible nature of the B4 and B5 sub-variants.

The major difference for red settings is in stricter gathering limits and the experts' view is that this would not make a marked difference because older people are being most affected by Covid-19 at present, says Ardern.

Instead the critical factor is vaccination, she says, adding that over 50s should get their booster shots.

"It's our older population at the moment where we are seeing the increase in those case numbers. We really need to make sure they have their booster to make a difference to our health system."

The older population are the age-group most likely to need hospital care.

She says it's not new measures that will make difference - it's an uptake of current ones, stressing again that people needed to wear their masks and get a booster.

Ardern says masks will also help restrict the spread of flu.

"It's the combination of winter illnesses ... that are having a huge impact on our hospitals."

Health experts have advised 50 plus for the second booster and not many countries have followed Australia's example in offering the second booster shot to over-30s, says Ardern.

"This is not a decision for politicians, it's our experts."

Asked if the health system is now unable to cope, she says that is not the case. Healthcare staff are doing "an incredible job" and are also being hit hard by illness or having to look after unwell family members.