A five-year-old from Boroko was confirmed to have the disease earlier this week, the third in the country since last month, Radio New Zealand International reported.
The manager for public health at PNG's health ministry, Daoni Esorom, said health teams had descended on the area to try and prevent any further cases.
PNG generally has a low vaccination rate, which allowed polio to re-emerge in the country in 2018.
Dr Esorom said he hoped a mass vaccination campaign that followed the polio outbreak had reduced the chance of a similar spread of measles.
"I cannot say we are not going to get any cases, but considering how measles can spread, you know, it takes almost three weeks.
"But from the way we are seeing things I'm cautiously confident we can attend to any measles outbreaks properly."