The country would not break legitimate exploration or development contracts or agreements, Prime Minister James Marape said in a posting on his Facebook page after the legislation was passed on Wednesday last week, Reuters reports.
"I can assure our investors that we know they must make money for their shareholders too, so we will not be greedy, but we (are) just asking for a fair share, if they want to harvest our resources," Marape wrote.
The government also plans to put in place a production sharing regime to take effect from 2025, he said.
Marape's push for more benefits for PNG has held up talks on Exxon's P'nyang gas development, key to a $13 billion plan to double the country's LNG exports.
Credit Suisse analyst Saul Kavonic said the amended law could pose challenges for the project.
"The move may signal the PNG government's resolve to maintain a tough approach to fiscal terms negotiations," Kavonic said in a note.
Exxon said in an emailed statement to Reuters: "We are hopeful that we can continue to work towards an outcome that benefits all stakeholders."
The mining law changes deal with the government's rights to take back tenements where a lease has lapsed or been cancelled.
That follows the government's refusal to extend the special mining lease over the Porgera gold mine, mainly owned by a joint venture of Barrick Gold Corp and China's Zijin Mining Group.
Newcrest Mining, which is looking to develop the Wafi Golpu gold project, said the changes would not affect its operations, Reuters reported.