Mr Bowen told the Australian Human Rights Commission inquiry into children in detention that he supported offshore processing as a deterrent to people smugglers, saying exempting children from the hardline policy would have encouraged people smugglers.

"Any particular groups of people would provide a significant incentive to people smugglers to load boats with that class of people, whether it be children [or] women," he said at the final hearing in Sydney on Tuesday.

"That was advice given to the government by various agencies."

Mr Bowen reopened the Manus Island centre in 2011 for children and adults. He had hoped they would be "open centres" where people would not be held in detention.

"My recollection is that very, very few children were sent to Manus Island under my ministerialship and that was because I was not yet satisfied that the appropriate care and support was put in place. That is my recollection," he said.

Children are not sent to Manus under the current government. There are currently 1084 single male detainees on Manus Island, according to the latest figures

Mr Bowen also said he often struggled maintaining role of guardian of unaccompanied children with his ministerial position , saying that he had considered  changing legislation to create distance between the minister and the consequences of the decisions they make.

"I had looked an alternative method," he said. "Ministers for Immigration make life and death decisions every day. It is a responsibility which I'm sure would weigh heavily on every minister."

Only five years ago another Labor immigration minister, Chris Evans, likened the responsibility of deciding the fate of asylum seekers to "playing God", saying he had too much power and the workload was too immense.

When questioned about the high levels of mental illness attached to detaining asylum seekers offshore, Mr Bowen also defended the policy, saying ministers had to weigh up the consequences of deterring people smugglers.

"I would say that almost everybody who has come to Australia by boat is at severe risk of being emotionally fragile and disturbed because of the obvious trauma they have already been through to cause them to come to Australia," he said.

"I fully accept, of course that detention is a very difficult environment to be in onshore or offshore."

But despite his support for offshore processing, Mr Bowen criticised the Abbott government for keeping children detained indefinitely on Christmas Island, saying he is concerned "as a citizen" for their treatment.

Mr Morrison has maintained there is no delay in transferring asylum seekers who arrived after July 19 to Nauru from Christmas island.

Mr Bowen was the first Labor minister to front the inquiry. Last month Immigration Minister Scott Morrison gave evidence in Canberra.

The report will be presented to the Immigration Department in the coming weeks.