Coronavirus is transmitted by droplets, often from the lungs.

Prime Minister Jacinda Ardern is advising anyone who has been overseas in the last two weeks to self-isolate, even if they got back before the official deadline.

Speaking to media on Wednesday, Ardern said the eight new cases were as a timely reminder for the need for travel restrictions.

"If you arrived before those border restrictions, seeing out the balance of your 14 days of self-isolation is the sensible, safest and best thing you can do for the community around you."

"We need New Zealanders to prepare. Think about Covid-19 the same way you would a civil defence emergency."

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"This is not a time for panic, it is a time for preparation."

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Currently, everyone arriving in New Zealand is required to isolate for 14 days after the Government tightened travel restrictions last week.

Hagen Hopkins/ Getty Images Prime Minister Jacinda Ardern has an important message for all international travelers arriving in New Zealand.

The tighter border controls were announced during an update by the prime minister on New Zealand's response to the Covid-19 outbreak on Saturday afternoon.

At the time she said while it is not realistic for New Zealand to only have a handful of coronavirus cases, the changes would slow the spread of infection.

On Wednesday a further eight cases of coronavirus were confirmed - bringing our national total to 20.

GETTY Director-General of Health for Ministry of Health, Ashley Bloomfield.

The new cases were in addition to the twelve people who had tested positive.

All these new cases announced on Wednesday are associated with overseas travel; four new cases in Auckland, two in Waikato, one in Christchurch and one in Invercargill.

Contact tracing for the people sitting near these people on the flights is under way.

The Ministry of Health expected more sporadic cases of coronavirus.

On Wednesday, director-general of health Dr Ashley Bloomfield said there were 30,000 swabs for coronavirus tests that were being distributed around the country.

Bloomfield has also said the ministry was ramping up testing, from about a 100 a day to 500.

"We have the capacity to do up to 1500 tests a day - well actually, we can do 750 to 1000 if we need to - but to go up to 1000, that just means doing an extra shift of laboratory staff. Again, we don't want to run our laboratory staff to the ground so what's most important is that we test the right people."

"I think that we have no evidence at the moment of community outbreak, and our border restrictions are intended to keep that risk low," he said.