Commissioners Prasart Pongsivapai and Narong Rathamarit visited Phuket on Saturday to obtain information from staff of the Department of National Parks, Wildlife and Plant Conservation.
Mr Narong said the NACC was getting involved because some parks department officials had filed a complaint against government officials alleged to have been complicit in the encroachment.
The commissioners received investigative reports from the Land Department and the parks department that alleged encroachment by Three Dolphins Co, which also owns the world-renowned Trisara luxury resort on the island.
The Land Department reported that at least five owners of land belonging to the national park, including Three Dolphins, possessed illegally issued title deeds for plots covering 463 rai.
The complaint given to the NACC accused six people supporting illegal issuance of land title deeds that resulted in the alleged encroachment. They were officials in the office of the petmanent secretary of the Interior Ministry, the Department of National Parks, Wildlife and Plant Conservation, the Land Department and some community leaders.
Mr Prasart said the NACC had also sought information from the Department of Special Investigation to support its own investigation.
Mr Narong said that the country had yet to plug legal loopholes that had allowed a large number of land rights documents be transformed into title deeds in a way that resulted in national forests being abused.
Investigations into illegal encroachment have been stepped up considerably since the military coup on May 22. While the problem has been well known for decades, especially in resort areas all over the country, enforcement of the law has generally been lacking given the clout and influence of those involved.