Papua New Guinea's "open" detention centre will be closed to an Australian senator when he arrives at Manus Island on a fact-finding mission on Tuesday.

Greens senator Nick McKim arrived in Port Moresby on Monday to discover his request to inspect the centre that still holds more than 800 men had been denied.

An aerial view of Manus Island in Papua New Guinea. Credit:Alex Ellinghausen

He branded the decision extraordinary, disappointing and frustrating, given the detention centre is fully funded by the Australian government and run by Australian contractors.

"I think it's unthinkable that such a ban would be imposed on Peter Dutton if he wanted to visit the camp, or any other Australian government official," he said.