Pressure is building on the Federal Government to resolve the position of asylum seekers in offshore detention, with the largest case yet against the Commonwealth to be mounted in the United Nations, claiming a policy of deliberately and permanently separating families is illegal under international law.

The Human Rights Law Centre (HLRC), which prepared the case in conjunction with a team of barristers, submitted it to the UN last week.

A group of 17 families — 70 people — will allege the Australian Government's policy of deliberately and permanently separating them is illegal under international law and will seek an urgent interim direction from the UN that they be immediately reunited.

The circumstances of the families are different: some were separated simply because they arrived on either side of the July 19, 2013 edict by then prime minister Kevin Rudd, that anyone arriving by boat from that date would never arrive in Australia; some have been separated because of medical evacuations to Australia.

'He missed the boat': Sudden policy change splits family

Soeharner's father, Nayser, is still on Nauru ( Supplied: Human Rights Law Centre )

In the case of the Ahmed family, the quirk of fate was the simple shortage of one extra seat in a minivan in Indonesia in 2013.

"We have to get on a small taxi, it only fit 11 people and we couldn't fit," Soeharner Ahmed told 7.30.

There was no room for her father, Nayser.

"He said he would come with the next car but the next car didn't come. They didn't arrange the next car so he couldn't come. That's how he missed the boat," she said.

Before Nayser arrived in Australian waters on a different boat, on July 19, the government changed the rules.

Now Nayser is stuck on Manus Island while the rest of his family are in Australia, with no prospect of a reunion.

Tina was medivacced from Nauru and gave birth to her son Mohammad in Australia ( ABC News )

In Adelaide, 18 month-old Mohammad has never met his father, Hani, who is stuck 4,800 kilometres away, also on Nauru.

Mohammad was born in Australia after his gravely ill mother, who requested to be called Tina for security reasons, was medically evacuated from the tiny Pacific island during her pregnancy.

Tina gave birth to her baby boy alone while her husband remained trapped on Nauru.

"My son is growing without his father and it's heartbreaking for him," she told 7.30.

"They have to bond with their father and this is what worries me — I want him to have a father figure."

Both families are taking part in the legal action in the hope they can be reunited with their loved ones.

"It's the last hope to reunite all of my family," Soeharner said.

"I did everything I can to bring him here, this is just my last hope."

Indefinite detention 'cruel and unbearable'

Hani is still on Nauru while his wife and child are in Australia ( Supplied: Human Rights Law Centre )

The HRLC's Daniel Webb said: "The impact of indefinite separation has been cruel, substantial and unbearable for each family.

"We're building a case that will allege that the Australian Government's policy of deliberately and permanently separating them is illegal under international law," he said.

The HRLC said the complaint would include comprehensive, independent psychiatric assessments detailing the acute medical harm being caused, especially to children, by the separations and the Government's failure to act.

It said the case would also outline tragic evidence taken from the Government's own medical files for family members, which contain increasingly dire descriptions of the deterioration in their mental health.

The separation from family members is directly linked to multiple suicide attempts.

These records also include repeated recommendations by the Government's own independent medical contractor, International Health and Medical Services (IHMS), to reunite the families in Australia.

There is also more pressure from the medical profession, with the Australian Medical Association delivering a letter to the Prime Minister signed by almost 6,000 doctors demanding the Government remove the 80 children from Nauru because of serious mental and physical health concerns.

The peak doctors' group's renewed push on the Government followed similar calls from Medecins Sans Frontiers (Doctors Without Borders) after the Nauru Government last week withdrew the NGO's capacity to provide mental health services on the island.

Political pressure mounting

Mohammad has never seen his father ( Supplied: Human Rights Law Centre )

The legal move comes as political pressure is growing on both sides of politics to find a resolution to the fate of about 1,500 people still on Manus Island and Nauru.

The issue is at play in the Wentworth by-election amid polls show while voters want a strong border policy they are increasingly uneasy about offshore detention.

An Essential poll last week found 37 per cent of voters believed the detention centre on Nauru should be closed and the remaining asylum seekers moved to Australia, and 40 per cent said families and children should be transferred to Australia.

Pollster Rebecca Huntley said feedback from focus groups showed Australians recognition that the conditions on Nauru and Manus Island were poor had increased.

"Some voters in some groups that surveys I've run say, 'it's a failed experiment'. They see offshore detention as not orderly, disorderly and dysfunctional as well as cruel," she told 7.30.

And there are increasing signs there will be a push on the federal Labor Party at its national conference in December to find a solution to the plight of the people still in offshore detention.

Immigration Minister David Coleman declined a request for an interview.

A spokesperson from the Department of Home Affairs said in a statement: "The Department does not comment on individual cases.

"Anyone who arrived in Australia by boat on or after July 19, 2013, will never settle in Australia," the statement said.

"The Department has repeatedly stated that individuals who have temporarily transferred to Australia, including for medical treatment, are expected to return to Nauru or PNG when the reason they transferred is complete.

"Medical transfer is not a pathway to settlement in Australia."