How do I dial on a mobile phone from Australia to PNG?
The same way you do from a fixed line telephone service.
Use the International access number - Telstra 0011 then the PNG country code - 675 -- then the telephone number. PNG does not have any STD codes. Instead of the International access cde most phone providers will allow you to use the "+" i.e. +675 for PNG
Of course this assumes your mobile telephone provider has given you overseas access.