The government also ordered the mandatory wearing of face masks in public in Port Moresby as part of a wide range of measures to help contain the spread of the disease.
Prime Minister James Marape was also quoted in local media as telling residents "not to be complacent or too relaxed because Covid-19 is spreading".
"All the health measures which have been repeatedly announced in the media must be taken seriously to save lives," he said.
According to reports, nightclubs will also be banned from opening but churches, restaurants and bars will remain open.
Having mostly avoided the Covid-19 pandemic until now, PNG had, by July 26, reported 62 cases of infection.
The first confirmed coronavirus-related death in Port Moresby occurred last Sunday, a 48-year-old woman had been previously diagnosed with breast cancer.
With limited coronavirus testing and many positive cases found in health workers, the authorities are concerned the virus might have a stronger foothold than the detected cases suggest.
National pandemic response controller David Manning expressed "serious concerns on the alarming rate of increase of Covid-19 cases in Port Moresby and the likely spread to the other provinces", and adding that there was a "high likelihood of expanded community transmission".
Its health system is already under severe pressure from the widespread transmission of malaria, tuberculosis and HIV/AIDS, as well as one of the world's few remaining outbreaks of polio.
Manning said the WHO was in the process of mobilising international emergency medical teams to deploy to the country and there was an urgent need for emergency health workers to help the country manage the outbreak.
"We are also discussing with the PNG Defence Force to assist ... the Health Department. We have always said we don't have adequate facilities."
Manning admitted testing had been very limited in areas outside the capital and while there was no evidence of hospitals being overwhelmed, that could be due to delayed reporting or poor health checking behaviour.
Last week, the WHO announced that it was scaling up testing of coronavirus in the country, providing additional laboratory supplies, including more than 1700 cartridges and 25,000 tubes for collection of coronavirus test samples.