One new case was confirmed in the National Capital District on Monday, at Port Moresby General Hospital, bringing the national total to 398.
The controller of the National Pandemic Response David Manning said there were too many people in the city not adhering to health protocols, despite the rising number of cases.
"The number of cases in NCD is now 254. This is a big increase and unless people adhere to the health measures in place, we are likely to see a continuing rise in cases in Covid-19,'' he said.
He urged every resident to wear a mask and practice social distancing.
Mr Manning also voiced concern at the situation across the border in Indonesia, where more than 2-thousand cases were confirmed in the last day alone.
He said the border with Indonesia remained a big threat to PNG.
Indonesian-administered provinces of Papua and West Papua have together reported 17 new cases in the past 24 hours, bringing the total confirmed cases for the two provinces to 4,177.