Two heavily pregnant refugees who refused to leave a bus for two days were intentionally left in the Northern Territory sun without air-conditioning and not brought food and water, advocates say.

After attempts to break the standoff failed, the women, both eight months pregnant, were removed with force and taken into detention, the Refugee Action Coalition said.

Spokesman Ian Rintoul said he had spoken with Adnam Shirvani who was on the bus with his pregnant wife Maryam and their 12-year-old son Amir.

"The department deliberately, intentionally moved the bus to keep it in the sun," Mr Rintoul said.

"They turned off the air-conditioning, allowing it to overheat. At one point both the pregnant women had fainted from the heat."

Immigration and Border Protection Minister Scott Morrison said he denied the claims of cruelty and mistreatment in the strongest possible terms.

Mr Rintoul also claimed the officials stopped bringing the refugees food and water, in an effort to get them off the bus.

"They said it was very obvious they were deliberately trying to force them off the bus."

Mr Rintoul described the conditions on the bus as extraordinary.

"There's legislation against keeping children or even animals in heated vehicles, so people can only imagine what kind of temperature it got to in the sun in Darwin," he said.

After initial attempts to coax the women from the bus failed, the heavily pregnant women were removed with force, Mr Rintoul said.

"He (Adnam) was forcibly held by at least four guards, and other guards went on the bus and dragged his wife out," he said.

"She was taken off the bus and spent the next 24 hours in the medical centre in the detention centre, separated from Adnam and Amir.

"She's extremely distraught."

Mr Rintoul said the family members were reunited on Tuesday night.

In a statement, a spokesperson for Mr Morrison described the claims as malicious.

"Health and welfare of the individuals was monitored throughout this incident," the spokesperson said.

"The individuals involved had access to food, water and amenities, and the Department and service providers facilitated access to toilets and health professionals."

The Minister confirmed both family groups were currently at Wickham Point detention facility.

Mr Morrison claimed both women went to the detention centre voluntarily.

"The Minister was advised at no time were the individuals told they would be accommodated in the community," the spokesperson said.

Refugees 'want to go back to Nauru rather than stay in detention'

Meanwhile Mr Rintoul said the refugees would rather return to Nauru despite its lack of medical facilities.

"The situation in Wickham Point is destroying his family and they want to be either put in the community or taken back to Nauru where they can prepare to have the baby free of the anxiety of immigration detention."

The two families were brought to Australia to give birth, which is standard practice for people who are having their application for refugee status processed in Nauru.

The ABC understands this was the first time someone who had been approved as a refugee had been flown to Australia to give birth.

The women have told refugee advocates they were not told they would be put back into detention once they arrived in Australia.

They claim they were told they would be housed in the community or a hotel.

Two of the Iranian refugees boarding a plane in Nauru bound for Darwin, according to the Refugee Action Coalition. ( Supplied: Refugee Action Coalition )

"They were not issued with travel documents or with visas either by the Australian Government or the Nauruan government, and when they arrived in Australia they were handed a bit of paper saying that they had arrived unlawfully in Australia and would not be taken into immigration detention.

"It's been all a lie from the Government."

After realising they were being taken to a detention facility they refused to leave the bus.

They offered to pay for their own accommodation and various refugee advocates offered to help find a place for the families to stay.

Their 12-year-old son was the first to leave the bus and went into Bladin Point detention centre, where he was believed to have other family members.

His father joined him soon after, but the two women and Tahere's husband Hossein remained on the bus until they were removed with force.

Both women were living with their families in the community in Nauru after they had been found to be genuine refugees and refugee advocates said they were terrified at the prospect of returning to detention.

The Department of Immigration and Border Security was slammed by refugee advocates and birthing associations for their treatment of the women.