More than half a million older New Zealanders are being asked to stay at home to avoid Covid-19 - though an exception will be made for the Deputy Prime Minister.

Prime Minister Jacinda Ardern today announced new measures in response to the growing risk of community transmission of the virus.

They included asking people over 70 years old, with compromised immunity, or respiratory conditions, to stay at home "as much as they can".

"That means we need friends, family and neighbours to support our older New Zealanders and people who may be in this group by doing simple things like keeping in contact and dropping off food or other supplies.

"And when you do, make sure you are not sick, that you are using good handwashing practices, and keeping your distance."

Census 2018 data shows there 528,000 people older than 70 in New Zealand, or around a tenth of the total population.

Among them was Deputy Prime Minister Winston Peters, who is 74. He is the oldest MP in Parliament and the only one captured by the new measures.

Ardern said Peters would continue in his role as "business as usual" because he was part of the Government's essential team, but he would be careful in terms of physical distancing. No one in Government would be travelling overseas, she said.

Up to 700,000 New Zealanders have a respiratory condition, most of them asthma which requires medication.

Asthma Foundation CEO Letitia O'Dwyer said her organisation was recommending that only people with chronic lung or respiratory diseases like emphysema and bronchiectasis stay at home.

That was still a large group of people. Around 15 per cent of over 45-year-olds in New Zealand fall into this category.

Those with mild asthma do not have to stay home, O'Dwyer said, but should keep their distance from people and keep their inhaler on them.

The stricter measures were introduced because of the growing risks of community transmission, which means the source of the infection is not known.

Director General of Health Ashley Bloomfield said earlier today that the total number of positive Covid-19 tests had risen to 52. Of that total, two could not be linked to overseas travel.

The ministry was now investigating how the two people, in Auckland and Wairarapa, contracted the virus.