It’s CASA PNG who make rules not ICAO and any country can choose not to apply an ICAO Standard, all they gotta do is file a difference. Australia and PNG have 100s of differences. Whatever is in Part 61 about ages and pairing is the bottom line. Any operator can choose to raise the bar on what the regs mandate.
As far as no competition on some of the international routes in and out of POM goes, pax are certainly using their feet to fly with other carriers and they are circumnavigating PX by travelling longer distances for cheaper tickets and better reliability.