The Minister for Civil Defence Peeni Henare announced the State of National Emergency following Prime Minister Jacinda Ardern's issuing of an Epidemic Notice on Tuesday.
New Zealand is attempting to eliminate the virus before it takes hold, having seen the devastation caused in countries such as Italy, where the authorities acted too late.
In a previous press conference, Ardern said the inevitable jump in cases should not be alarm Kiwis.
Ardern said New Zealand could expect to have "several thousand" more cases of coronavirus in New Zealand before strict self-isolation of the whole population had an effect on transmission.
Director-General of Health Ashley Bloomfield said numbers would continue to rise as households began to mandatorily self-isolate under tough new government restrictions.
Civil Defence Minister Peeni Henare speaks during question time at Parliament on December 11, 2019 in Wellington, New Zealand.
This provided access to special powers to combat Covid-19 - including powers of requisition and closing roads, and stopping people from doing certain activities.
"An influx of people who don't have tickets for a crossing are turning up", police said, advising those without bookings to stay away.
'Each of you has a role in helping to save a life, ' she said.
Yesterday the number of confirmed and probable cases rose to 155, including 12 who had recovered and six who were in hospital but have not needed ICU treatment.
Whangaparoa, the military base previously used to quarantine those New Zealanders flown home from the virus' previous centre in Wuhan, China, would be again used for incoming Kiwis.
'There is a lag (between) the transmission and the manifestation of COVID-19 symptoms.
Ardern urged people to "stay local" else the lockdown may be extended.
Acting district commander Inspector Darryl Sweeney said police would increase their visibility in the community for the month-long lockdown to provide reassurance but were also working with the owners of businesses to provided added security.
There is support for the Government's new quarantine rules. The Police will work with New Zealand Defence to ensure that essential services are provided without hinderance.
The Reserve Bank has reduced the Official Cash Rate to 0.25%, relaxed capital requirements of retail banks and purchased government bonds valued at $30 billion to maintain currency flow, while retail banks have also stepped their levels of assistance.