The Independent Inquiry into Child Sexual Abuse will hold its first public hearings on Monday.
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 six million-pound Family Restoration Fund was set up to allow the migrants to travel to the UK and ministers are now considering extending it.
The independent inquiry was set up after the death of DJ Jimmy Savile in 2011 when hundreds of people came forward to say he had abused them as children.
The spotlight then fell on sexual assaults carried out in schools, children’s homes and at NHS sites, as well as on claims of past failures by police and prosecutors to properly investigate allegations.
But since being set up by the then Home Secretary Theresa May, it has been marred by a number of issues.
Its first chairwoman, Baroness Butler-Sloss, gave up the post after just one week because concerns arose around her links to the establishment – namely her late brother, Sir Michael Havers, who was attorney general in the 1980s.
Lord Mayor of London Fiona Woolf was appointed in September 2015, but resigned less than two months later after disclosing she had been to five dinners with the late Lord Brittan – one of the people facing accusations at the time, which have since been dropped.
And Justice Lowell Goddard held the post for 18 months, but quit in August 2016 due to “compounding difficulties” and her family life.
Even the current chairwoman, Prof Alexis Jay, has also faced criticism, including from the Shirley Oaks Survivors Association, that pulled out of the inquiry after saying it had lost confidence in its leadership.
But she vowed to continue in the job, despite calls for her to quit.
A number of lawyers have also resigned or been removed from the process.
The hearings will take place at the International Dispute Resolution Centre in London, with the first phase concerning Australia expected to last 10 days.