As I understand, form the general statement of IRS, "If you are a U.S. citizen or resident alien, the rules for filing income, estate, and gift tax returns and paying estimated tax are generally the same whether you are in the United States or abroad. Your worldwide income is subject to U.S. income tax, regardless of where you reside."
Check with your IRS if there is tax treatise between PNG and USA so that you will not be subject to double taxation.