At least 20 domestic workers have escaped exploitation in diplomatic residences, including having been made to work 12 to 18-hour days for a fraction of the minimum wage and being forbidden from leaving the premises, an ABC investigation has found.

Key points: Four Corners has spoken to three workers who were kept in conditions they liken to slavery by diplomats from three different embassies in Canberra

Four Corners has spoken to three workers who were kept in conditions they liken to slavery by diplomats from three different embassies in Canberra The Salvation Army's Jenny Stanger says she has seen "over 20 cases coming from diplomatic households"

The Salvation Army's Jenny Stanger says she has seen "over 20 cases coming from diplomatic households" The international legal concept of diplomatic immunity means foreign diplomats are not subject to Australian law

Four Corners has spoken to three workers who were kept in conditions they liken to slavery by diplomats from three different embassies in Canberra, but because of diplomatic immunity, Australian authorities remain largely powerless to act.

"It's incredible to think that in the heart of Australia, that these sort of 19th century practices are taking place," said David Hillard, pro bono partner at the large law firm Clayton Utz, who acts for some of the escaped workers.

"Slavery is happening around the world and Australia is no exception," one of the workers told Four Corners.

"I was like in a prison cell, like in a box or in a room — all you see is the four corners of it every day," she added.

Australian government guidelines require that all domestic staff in diplomatic households keep their passports at all times, but two of the workers the program spoke to say they had their passports removed by their employers.

The Salvation Army's Jenny Stanger has worked with exploited domestic workers for a decade.

"We've had over 20 cases coming from the diplomatic households, primarily in Canberra," Ms Stanger said.

Jenny Stanger says she has seen more than 20 cases come through the Salvation Army. ( ABC News )

One of the diplomats is still in her job in a very senior post, despite her former domestic worker being granted a protection visa following Australian Federal Police and Immigration Department investigations, because it is unsafe to return to his home country after he escaped her residence.

The workers Four Corners interviewed were given contracts in their home countries which were vetted by the Department of Foreign Affairs and Trade, saying they would be paid the minimum wage and they would meet Australian workplace laws.

But when they got to Australia, one was told to ignore that contract and they were all paid a fraction of the salary the contracts stipulated.

Each of them said they were never given weekends off, had no holidays and no time in lieu for the overtime they all did every day, working up to 18 hours a day.

"I think it shows a real clear intention to disregard our laws and our expectations of what it means to employ someone here in Australia," Ms Stanger said.

Diplomatic immunity means foreign diplomats can't be brought before Australian courts.

Dr Alison Pert, a specialist in international law from the University of Sydney, says introducing exceptions to diplomatic immunity might mean that diplomats from Australia be liable to the same treatment elsewhere.

"So, the options are really moral or political — they're not legal," Dr Pert said.

They could include the sending state waiving the diplomatic immunity, removing the official from their post, or encouraging them to pay the money that is owed to the domestic worker.

Foreign Minister Julie Bishop said in a statement that her department "treats allegations of mistreatment of domestic workers by foreign diplomats very seriously and they are a matter for the police".

Watch Behind Closed Doors on Four Corners at 8:30pm.