Former ABC managing director David Hill has told a UK inquiry the British government betrayed thousands of child migrants who were sexually abused, beaten and used as slave labour at Australian farm schools.

The 70-year-old told the child sexual abuse inquiry sitting in London that former prime minister John Major's assertion that child migrants were not the British government's responsibility once they were sent away was "disturbing".

At age 12, Mr Hill was sent to the Fairbridge Society farm school at Molong in NSW in 1959 along with other children who had been put into care.

His 2007 book The Forgotten Children details the routine abuse suffered by children there.

In May, the inquiry heard former child migrants tell of the sexual, physical and emotional abuse they suffered at institutions run by Fairbridge, the Christian Brothers and other church and charity groups in Australia up to the 1970s.

In a statement to the inquiry last week, Mr Major said his position when prime minister from 1990 to 1997 was that any allegations of abuse of child migrants in Australia "would be a matter for the Australian authorities".

But Mr Hill, his voice breaking with emotion, took issue with that stance on Wednesday.

"John Major's assertion that once we children left we became the responsibility of a foreign government and not the responsibility of the British government is disturbing," he said.

"We were still British children, we were still British citizens deserving protection of the British government."

Mr Hill said evidence before the inquiry showed his earlier calculation that 60 per cent of children at Molong were sexually abused was likely an underestimate.

He said migrant children as young as four had been taken from their parents and shipped overseas into the care of "unqualified, sadistic and poorly selected staff" in institutions run on rigid Edwardian standards.

Children lived in conditions harsher than in adult prisons, were poorly clothed and fed, ate off metal plates and bowls and slept on beds without pillows, Mr Hill said.

"The boys qualified for nothing more than cheap farm labourers and the girls as domestic servants," he said.

Mr Hill said the British government "betrayed" migrant children by failing to ensure their safety and welfare and failing to act even when officials knew of the sexual and physical abuse that was taking place.

"I know of no other country that abandoned tens of thousands of its defenceless children simply because they were poor," he said.

Mr Hill called for the care institutions involved to urgently give full apologies, compensation, support and counselling to surviving former child migrants before they died.

The inquiry concluded its public hearings on Wednesday and will hand a report to the UK government.