It will start by examining the cases of British children sent to Australia between 1945 and 1974.
The inquiry will eventually investigate claims against councils, religious organisations, the armed forces and public and private institutions.
The inquiry, set up in 2014, has been dogged by controversy and is now on its fourth chairwoman.
Between 7,000 and 10,000 British children from poor families and the care system were sent to live in Australia after World War Two.
They were recruited by religious institutions from both the Anglican and Catholic churches, or well-meaning charities, including Barnardo’s and the Fairbridge Society, with the aim of giving them a better life.
Many, however, went on to suffer physical and sexual abuse in homes and so-called farm schools run by religious orders and charities.
The BBC’s home affairs correspondent Tom Symonds says the inquiry will be told that the scale of sexual abuse they suffered was much wider than previously thought.
In 2009, the Australian government apologised for the cruelty shown to the child migrants. Britain also made an apology in 2010.
A £6m family restoration fund was set up to allow the migrants to travel to the UK and ministers are now considering extending it.