Direct answer
Foreign travellers should assume they need an appropriate Entry Permit unless a current ICA rule gives them another entry facility
Papua New Guinea does not operate a broad, open visa-free tourism policy. Instead, foreign nationals enter through visa or Entry Permit classes, visa on arrival where ICA currently permits it, or specific arrangements such as the APEC Business Travel Card procedure.
The word “visa” is commonly used by travellers, but ICA's underlying immigration framework uses Entry Permits. The route used to obtain that permission depends on the passport and the activity to be carried out in PNG.