An advocacy group in Papua New Guinea says a proposed leasing system could leave customary owners landless.
Act Now's programme manager Effrey Dademo said the government was planning to convert illegal Special Agricultural Business Leases into registered land which it could sublease.
A 2012 Commission of Inquiry found the majority of the leases, mainly used for logging, were obtained illegally.
But Ms Dademo said the government's plan would deny customary landowners their constitutional right to decide what happens.
"The land is under customary tenure. This is what people depend on. You're converting land, you're registering it which means there are questionable leaseholders having access to these lands and then you are registering it in their name, it becomes a perfect title for them and these landowners are not going to get their land back. That's our concern."
More than five million hectares of land is estimated to have been affected.