Asylum seekers, including mothers and their children, are spending nearly five years in Australian immigration detention facilities, as the time in detention has soared to one of the longest rates in the world.

The Commonwealth and Immigration Ombudsman has slammed the government for its prolonged detention of asylum seekers after finding children are being born, and remain, in detention facilities waiting for their families' claims of protection to be processed or for their security assessments to be cleared.

According to the Ombudsman's assessments, one 33-year-old woman known as "Ms X" and her three children, "Master Z", aged 17, "Master Q", 14, and "Miss W", 10, have remained in community detention for four and a half years, or 1643 days. The family has been found to be owed protection by Australia, but the mother is awaiting her "security assessment", the report says.

The May 27 report, which is designed to be a "snapshot in time" of immigration detention, also shows six children had been born in detention between November 2012 and February 2013. All six children live in community detention with their families, but not one of their families' claims have been processed, it says.

"The Ombudsman notes with concern the government's duty of care to detainees and the serious risk to mental and physical health prolonged detention may pose to children and their families," the report says.

"Without an assessment of claims of these children and their families to determine if they are found to engage Australia's protection obligations, it appears likely that they will remain in detention for an indefinite period."

A 37-year-old man spent 1827 days in detention, which is more than five years. He was released from immigration detention in December last year on a bridging visa without the ability to work.

During Senate estimates last week, it was revealed that longest time in detention for a child was 1774 days, which is nearly five years.

A spokesman for the Darwin Asylum Seeker Support and Advocacy Network, DASSAN, said he was aware of a number of families who had been kept at Wickham Point in Darwin for up to four years.

"Those families are not told why they are still in detention or what can be done to secure their release," Ben Pynt said.

Under former immigration minister Scott Morrison, the processing of claims was paused until the amendments to the Migration Act were passed. They were passed in December last year.

A spokeswoman for Immigration Minister Peter Dutton said that processing had now begun and "several individuals" in the Ombudsman's report have since been granted temporary visas and were living the community. The minister did not say which cases had been dealt with.

According to the estimates hearing, of the 30,448 asylum seekers who arrived by boat under the Labor government, only 600 have begun to have their protection claims assessed. According to Michael Manthorpe, deputy secretary of the Visa and Citizenship Services Group, it could take three years for these claims to be processed.

The latest Immigration statistics also show 1195 people have remained in held detention and community detention for longer than 730 days, or two years.

But Mr Dutton's spokeswoman said a new "fast-track assessment" process would allow officials to "quickly and appropriately manage different types of claims".

Grant Mitchell, director of the International Detention Coalition, said Australia had one of the longest immigration detention rates in the world.

"The big problem in Australia is that there is no time limit to how long people can be kept in immigration detention," he said.

"Australia is unique that it lacks court oversight on decisions, leaving it in the hands of the minister's discretion, which is highly problematic," he said. "We are lagging far behind."

In Sweden, asylum seekers can be held in immigration detention for a maximum of five days, while a child can be held for only three days under Swedish law, Mr Mitchell said. He also said New Zealand recently adopted laws that limit the duration of immigration detention to 14 days before a judge must make a decision.

Greens Senator Sarah Hanson-Young said children should not be incarcerated.

"Children don't belong in detention and it is time the law was changed to ban it from happening," she said.

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