COVID-19 cases will rise for at least 10 days before declining - officials

New Zealand's top health official warns there will be a rise in coronavirus COVID-19 cases for at least the next 10 days.

Director-General of Health Dr Ashley Bloomfield says a turnaround will only happen if people follow lockdown rules.

Dr Bloomfield on Wednesday announced 50 new confirmed or probable cases in New Zealand with five now confirmed at Auckland's Marist College.

"I would say that even as we go into alert level 4, we will continue to see a rise in cases for at least 10 days," he told reporters. 

"The numbers will continue to increase before they turn around. That turnaround will happen if all New Zealanders do what is being asked of them; that is to stay at home, to break the chain, and save lives.

"Look after yourselves, and look after other people."

There remain four confirmed cases of community transmission of COVID-19 with several other suspected cases, Dr Bloomfield said.

Addressing Parliament on Wednesday afternoon, Prime Minister Jacinda Ardern urged New Zealanders not to be complacent.

"Make no mistake, this will get worse before it gets better. We will have a lag and cases will increase for the next week or actually more.

"I'm fully aware that we've moved with huge speed - no other country in the world has moved to these measures with no deaths and infections at the level we have.

"But we have no time to waste - every hour we wait is one more person, two more people, three more people, exposed to COVID-19."

Opposition leader Simon Bridges showed unity in Parliament, saying getting New Zealand through the crisis was about working as a team.

The country will move into COVID-19 alert level 4 at midnight - effectively placing it into lockdown.