Hello Wantok,
I think you should have a look at this page/site
http://www.customs.gov.pg/index.html
and get it writing from customs, and keep a copy of the email and your contact at customs for when the vehicles hits PNG.
The general rule is if you can show ownership of the vehicle including insurance papers for 12 months or more the vehicle should come into PNG screen free, but there will quarantine fees.