The Commonwealth has agreed to pay $70 million in compensation to more than 1,900 asylum seekers who spent time on Manus Island between 2012 and 2016.

It has agreed to an in-principle settlement of the case brought by the asylum seekers, which was set to begin in the Victorian Supreme Court today, in a case which had been years in the making.

Expected to run for six months, the case would have examined conditions in detention in a way that up until now had not been possible, due to strict access restrictions.

Why were asylum seekers suing?

The asylum seekers alleged the Commonwealth breached its duty of care by holding them in conditions which did not meet Australian standards and caused them physical and psychological harm.

They also claimed they were falsely imprisoned after PNG's Supreme Court ruled their detention was illegal as it breached the right to personal liberty in the country's constitution.

They also alleged the Commonwealth knew they were likely to suffer and deliberately disregarded their rights and breached international conventions.

The group was also suing security companies G4S and Transfield which were contracted to run the processing centre.

They alleged the Commonwealth and the centre's operators failed to make sure there was an adequate supply of food, water, shelter, medical treatment, personal hygiene products and security to ensure the asylum seekers were protected from outsiders trying to gain entry or other detainees considered dangerous.

Included in the allegations were that:

Detainees were at times provided with food contaminated by worms, maggots, flies other insects, human hair, teeth or sweat

Detainees were at times provided with food contaminated by worms, maggots, flies other insects, human hair, teeth or sweat Detainees often had to queue for hours to receive meals each day

Detainees often had to queue for hours to receive meals each day Detainees had to request toilet paper on an as-needs basis

Detainees had to request toilet paper on an as-needs basis Detainees seeking medical treatment routinely waited three days or more for a medical appointment, regardless of their condition

Detainees seeking medical treatment routinely waited three days or more for a medical appointment, regardless of their condition The medical centre did not have a permanent or full-time psychiatrist despite 30 per cent of detainees presenting with some form of mental health issue

The medical centre did not have a permanent or full-time psychiatrist despite 30 per cent of detainees presenting with some form of mental health issue In a number of cases, authorities refused to transfer detainees to Port Moresby or Australia for further medical treatment

In a number of cases, authorities refused to transfer detainees to Port Moresby or Australia for further medical treatment G4S did not adequately respond to reports the perimeter fence was inadequate to stop asylum seekers attempting suicide by drowning themselves in the sea; and

G4S did not adequately respond to reports the perimeter fence was inadequate to stop asylum seekers attempting suicide by drowning themselves in the sea; and Detainees had limited access to condoms despite a number having sexually transmitted diseases.

In its response to the asylum seekers' claims, the Commonwealth denied many of the accusations and argued that duty of care was delegated to an operational manager from the PNG Immigration and Citizenship Service Authority.

It said the detention centre was under the control and management of that officer during the relevant period.

G4S and Transfield were also fighting the case.

Who was at the centre of the case?

The asylum seeker who was leading the class action was 35-year-old Iranian man Majid Karami Kamasaee who was detained on Manus Island for 11 months from September 2013, and remains in a Melbourne detention centre

Mr Kamasaee alleged he was persecuted by Iranian authorities for converting to Christianity and was forced to study the Bible in secret.

He claims to have fled the country under threat of being jailed for his religious beliefs.

He attempted to reach Australia by boat from Indonesia but it was intercepted by the Navy and he was transferred to the offshore processing centre.

Mr Kamasaee suffered serious burns to his face, neck and hands in a heater fire at his home in the Iranian capital of Tehran when he was about 15.

He required more than 30 surgical procedures to treat his injuries which continued to cause him ongoing pain and skin sensitivity.

While Mr Kamasaee was detained on Manus Island, he alleges he experienced severe pain and irritation to his skin as well as other injuries.

According to a statement of claim, his skin creams were confiscated by the Commonwealth when he was detained and they were never replaced.

Mr Kamasaee alleges the hot and humid conditions on Manus Island caused serious itching, pain and irritation and caused growths to develop on his face.

Medical staff advised him to apply sunscreen, Vaseline or non-medical Nivea skin cream instead, according to court documents.

He remained at the centre until July 2014 when he was transferred to Melbourne for medical treatment.

Why is this class action important?

The class action was set to examine conditions at the offshore processing centre on Manus Island in forensic detail and hear from more than 100 witnesses including asylum seekers and staff.

Two men were found guilty of murdering asylum seeker Reza Barati while he was in detention on Manus Island. ( AAP: Dan Peled )

It was also slated to cover the deadly February 2014 riot in which 23-year-old Iranian asylum seeker Reza Barati was killed.

Law firm Slater and Gordon, which ran the class action, believed it to have been the largest immigration detention trial ever in Australia.

The Victorian Supreme Court agreed to live stream the proceedings because of the high level of public interest and considerable number of asylum seekers involved in the trial but currently living overseas.

But the court had previously commented that the proceeding was not intended to be "a commission of inquiry into the merits of the Commonwealth's offshore detention policy".

Where are the asylum seekers now?

The asylum seekers involved in the class action are now spread across the world.

On the latest count, 856 remained at the Manus Island detention centre while 103 lived elsewhere in PNG.

Six asylum seekers involved in the proceeding are in Nauru, 54 are in immigration detention in Australia, 11 are in community detention and one is in criminal detention.

Almost 400 are living in the Australian community on a bridging visa or waiting for their status to be determined.

More than 500 group members had been in the care or in contact with the International Organisation for Migration to return to their country of origin.

And 425 detainees have done just that, with most returning to Iran and smaller numbers going back to Vietnam, Iraq, Lebanon, India and several other countries.