I presume you will be living and working in Port Moresby ?
I have worked in PNG as civil superintendent on remote sites.
The last site was Komo air port,2011, and my take home salary was AUS$12,000 on a 6 week on 2 week off roster.All medical, travel etc paid for by company.
You state schooling paid for so you will not be fly in fly out but residential. Does the company cover superannuation ??If not best you include that in the negotiations.
Engineers on site at Komo, with little overseas experience, got $10,000 a month net.
Hope this helps.