The head of Amnesty International claims the human rights group has been barred access from inspecting conditions at the Nauru immigration detention centre.

Secretary general Salil Shetty says Australian and Nauru authorities have denied access to Nauru following Amnesty's 2013 visit to the Papua New Guinea Manus Island detention centre where it documented overcrowding, ill health, and violence against the asylum seekers.

"If there is nothing to hide, why are we not allowed in?" Mr Shetty told ABC TV, ahead of his visit to Australia next week.