Immigration Minister Scott Morrison has blamed Labor’s legacy for a trebling of the average time children are kept in detention.

During a fiery session at the Human Rights Commission inquiry into child detention in Canberra, Mr Morrison said effective immigration policy was always going to come at a cost.

• Morrison to front asylum inquiry
• EXPLAINED Detained children to be released

It was revealed asylum seekers spent an average of 349 days in onshore detention as of July this year, more than triple the average of 115 days when the government came to power in September last year.

Mr Morrison blamed a “legacy caseload” and said previous Labor policies were “a source of great frustration”.

“It would have been my preference that at the beginning of this year that we would have got underway with processing the 30,000 people onshore that were left behind by the previous government,” he said.

When questioned by commission president Gillian Triggs on what evidence the government had that holding children in detention for more than a year was stopping boats, Mr Morrison said: “We are doing what we said we would do.”

“Our policies are getting results.”

In his opening remarks, the minister suggested that he predecessors should also be invited to speak at the commission.

During questioning, Mr Morrison rankled at Ms Triggs’ characterisation of detention centres as “prisons”.