The Prime Minister announced Cabinet's decision on Monday and said once alert level 3 is in place, it will be reviewed again with the decision to move into alert level 2 on May 11.

"Going hard and early to stamp out the virus, backed by an effort of 5 million New Zealanders to break the chain of transmission, means together we have helped stop the uncontrolled explosion of COVID-19 in New Zealand," Ardern said.

The Director-General of Health Dr Ashley Bloomfield said he is confident there is currently no widespread undetected community transmission in New Zealand, which played into Cabinet's decision about the lockdown.

"Cabinet wanted to make sure we lock in our gains, give ourselves some additional certainty, but take no more than two additional business days to do this before we restart our economy again," Ardern said.

At alert level 3, key sectors will return to work, such as construction, manufacturing and forestry, and Kiwis will be able to order takeout food online. But you will not be able to go to the gym, get a haircut or go out on your boat.

A reminder of the principles for level 3

If you are not at work, school, exercising or getting essentials, then you must be at home

Work and learn from home if you can

Make your business COVID-19 safe

Stay regional and exercise at parks or beaches within your region

Keep your bubble as small as possible but expand it if you need to

Keep practising good hygiene such as washing your hands often with soap

If you are sick stay at home and seek advice from your GP or Healthline about getting a test

"At level 3 we need to be even more vigilant and we must stick to the rules, continue to reduce contact and keep breaking the chain of transmission," Ardern said.

"And remember that we remain at level 4 until you wake up next Tuesday. Let's stick with our plan and the mission we have. Stay strong, stay home, let's finish what we started."

Under alert level 3, children who can stay home should stay home, the Government has advised, and at-risk students and staff should also stay home, and will be supported to do so.

Early learning centres and schools will physically be open for up to year 10 for families that need them, and people will be allowed to enter schools and early learning centres from Tuesday, April 21, for cleaning, maintenance and other preparations to re-open.

The Government expects most children who need to attend school to be able to do so from April 29, but it may take longer for some schools and early learning centres to be open.

Alert level 3: What you can and can't do