The United Nations has reportedly called for an independent mechanism to monitor Australia's detention centre on Nauru.

The president of the UN's sub-committee on torture prevention, Malcom Evans, is urging for the provision after inspecting the centre this week.

The Guardian newspaper says UN representatives were allowed to photograph and interview asylum seekers away from the presence of guards during the visit.

The move follows suggestions by UN special rapporteur on torture, Juan Mendez, that prolonged detention like that on Nauru could contravene international conventions.

"Given the number of people currently being held on the island, the establishment of a national preventative mechanism to address their needs and their situation becomes even more pressing," Evans said.

The call also comes a day after Immigration Minister Peter Dutton announced the imminent transfer of the first batch of Nauru detainees to Cambodia.