Prime Minister Jacinda Ardern said Monday was the day to ramp up the nation's response to Covid-19 to stop its spread.
She said whilst New Zealand was someway behind other countries in terms of the number of cases, the country needed to act quickly to contain it.
Ardern said now is the time to act and the only way to reduce the spread of the virus was for people to stay at home.
She went on to explain what alert levels three and four mean.
Schools would be closed from Tuesday, Ardern said. She said travel around New Zealand would also change.
New Zealand will be under alert level four - the highest level - from Wednesday, when the country would be in lockdown.