The public health warning is especially pertinent if someone is planning to go to Pasifika or the March 15 memorials this weekend, Ardern said.
"[It's] incredibly important. If you are sick do not go.
"If you are sick, do not go to work. If you are sick, do not go to mass gatherings. That is one of the most important messages we need to get out.
"You might not feel that unwell, but you're putting at risk others."
Ardern also said there would be no immediate change to the Government's response to the Covid-19 outbreak in light of the World Health Organisation declaring it a pandemic.
This is because the New Zealand Influenza Pandemic Plan had already been activated in January, Ardern said.
"From a practical perspective for New Zealand, we've already been treating Covid-19 as a pandemic."
The self-isolation requirements for tourists from coronavirus hotspots has seen travel effectively "turned off" from those countries, like Italy and South Korea.
"In some cases, looking at the numbers, in my recollection, fewer than five, and that would include permanent residents and citizens, so that has had that effect of people having to rethink any travel."
The Prime Minister spoke to media this morning at the Island Bay Medical Centre in Wellington after being given a tour of the clinic with Health Minister David Clark.
The medical centre is putting up a perspex screen at reception to protect receptionists from coughing patients.
They're also about to put a cabin at the back of the practice which can be used as a video conference area where sick people can sit without coming into contact with staff.
The ministers talked with its staff for more than 30 minutes about how they were responding and preparing to the virus.
GP Dr Richard Medlicott said Ardern and Clark asked how the Government could be doing more.
The doctor said he told her there needed to be money available for costs incurred, like their shed and more investigation into technical options like video conferencing.
Medlicott said he also told Ardern and Clark they had to ensure clinics, especially in lower socio-economic areas, were equipped to handle a community outbreak as many were overrun during the swine flu pandemic.