The Governor of Papua New Guinea's capital Port Moresby has taken out full-page advertisements in local newspapers criticising the Manus Island detention centre.

The open letter from Governor Powes Parkop to PNG's Foreign Minister Rimbink Pato expresses "grave concern" at the treatment of asylum seekers on Manus Island.

It says asylum seekers have not committed a crime and should not be held in prison-like conditions.

"This is an Australian practice which we should guard ourselves against," wrote Mr Parkop.

"It is... repugnant to our traditional and contemporary culture and to our Christian values to keep such people in near prison-like environment."

"Such environment and conditions can make normal and decent people incur mental problems, including insanity."

The letter says Papua New Guinea is "blindly" following Australian policy, which Mr Parkop says goes against the principles of the UN Convention on Refugees.

He proposes instead that asylum seekers with professional qualifications be given work permits while they wait for their refugee status to be assessed.

"I understand our people are opposed to settlement in PNG but I believe this attitude is influenced by the perception that some of these asylum seekers might be extremist or Muslim fanatics or troublemakers," Mr Parkop said.

"While I don't wish to ignore such possibility … many of these people might be fleeing that type of environment too.

"Let us not demonise these people forever."

In his open letter, Mr Parkop says many nations have been enriched by immigration, including migration motivated by economic reason.

He cites the Irish fleeing famine and migrating to the United States of America as an example, as well as Chinese, Indian and Filipino migrants worldwide.

"I am under no illusions that this is a delicate and sensitive matter but I believe we can and should adopt a more human and morally superior approach that adopting Australian policy and culture or be blinded by our people's fears and prejudice."

The letter was also sent to PNG's Prime Minister Peter O'Neill and the Australian High Commissioner, Deborah Stokes, and was published as a full-page advertisement in two newspapers.