What counts as an overstay?
Your authorised stay ends when the Entry Permit conditions say it ends
PNG Entry Permits are issued for a stated purpose and period. Once the permitted stay expires, remaining in Papua New Guinea without another valid immigration basis can make the person unlawful.
Do not calculate the deadline from assumptions such as “one month equals 30 days” or “the visa is valid until the end of the calendar month.” Use the actual Entry Permit grant, border endorsement or ICA record that applies to the traveller.