The number of people in immigration detention must be urgently reduced to address COVID-19 risks, the Australian Human Rights Commission says.

An outbreak of COVID-19 on Christmas Island could be catastrophic and the island should not be used for immigration detention, the Australian Human Rights Commission has warned.

Shuttering the detention facilities on the island - which hold a Tamil family of four and more than 200 men - is one of 20 recommendations put forward by the commission to address the risk of a disease outbreak.

A coronavirus outbreak on Christmas Island "could be catastrophic", Human Rights Commissioner Edward Santow told AAP.

"If there were an outbreak it would be very difficult to treat the people affected, but it would also be very difficult to limit the spread on the island itself."

There's no acute medical care available on the island and no ventilators. The closest hospital is 2500km away in Perth.

COVID-19 swabs are flown to Perth to be tested.

The island's North West Point detention centre was put back into action in August 2020 to cope with the pressure on the mainland facilities. Some 217 men were held there at the end of March.

The Murugappan family had previously been the island's only detainees.

The urgent evacuation of three-year-old Tharnicaa Murugappan to Perth last week "just shows that you cannot get good care on Christmas Island," infectious diseases specialist Nadia Chaves told AAP.

Dr Chaves assisted the commission's review of the management of COVID-19 risks in immigration detention as an independent expert.

The report, published on Wednesday, sounds the alarm over overcrowding and a lack of physical distancing within Australia's detention facilities.

The Trump administration oversaw a 69 per cent reduction in the number of people in immigration detention between October 2019 and December 2020.

Canada and the United Kingdom also drastically reduced their detention headcounts during the pandemic.

But the number of detainees in Australia has risen - up 11.7 per cent between March and October 2020.

The population must be urgently and significantly reduced, the report states.

Australia's approach "doesn't make sense", said Dr Chaves. "It's an unnecessary risk."

She said it was impossible to stay at 1.5 metres' distance from other detainees, based on reports and information she had been provided by the government.

Public health experts have been "crystal clear" that the risk of COVID-19 in detention settings is reduced by allowing physical distancing, Mr Santow said.

The easiest way of doing that would have been to reduce the detainee population.

More than one in ten people in detention - 247 of more than 1500 - have conditions or are of an age that makes them particularly vulnerable to COVID-19.

Mr Santow says that if those people can't be released, they should be held in single rooms with separate bathroom facilities.

The commission is also concerned about the "prison-like" conditions that refugees, asylum seekers and other detainees are held in when they're put through a 14-day quarantine.

It says that some people are held in the harsh rooms for two weeks just for attending medical appointments outside the detention centre, even if they're in a low-risk part of the country.

The areas used for quarantine have no or very limited access to natural light, fresh air and outdoor space.

"In states where there was no COVID, still people were being quarantined for 14 days in solitary confinement essentially, regardless of symptoms," Dr Chaves said.

"It's very restrictive and it has an impact on mental health."

No immigration detainees have so far tested positive for COVID-19, though several guards have.

The report shows that people are staying in detention for longer, with the average detainee held for 616 days by December 2020.

That's the highest the average has ever been.

More than 100 people have been held for more than five years. Some have been detained for over a decade.

Of the commission's 20 recommendations, the government has accepted eight in whole or in part, disagreed with five and noted seven.