At least 60 Australians have been listed among more than 300 witnesses.
The inquiry will look at allegations of physical, sexual and emotional abuse over a period of 70 years at homes and institutions run by state, church and voluntary organisations.
Chairman Sir Anthony Hart opened the inquiry and noted there were some who had waited years to give their evidence.
Among those giving evidence were 61 people from Australia.
Caroline Carroll, Chair of the Alliance for forgotten Australians, said the need was great.
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"I know there was a huge amount of interest from people who were in Ireland - so much so that two commissioners had to come to Australia to meet with people," she said.
Thousands of children from the United Kingdom were sent to homes in Australia and their treatment here was often just as harsh.
In 2009, then-Prime Minister Kevin Rudd issued an official apology.
Adults Surviving Child Abuse president Cathy Kezelman said the victims’ trust had been betrayed.
"It was purported to be in the children's best interest but often they were lied to, told they were orphans and dislocated from their homes, families and identities," she said.
The inquiry is looking at 13 institutions and some are calling for that to be broadened. It will take 18 months, and the final report is due in 2016.
If you are child migrant who would like more information on the inquiry, contact the Child Migrants Trust Inc, free call: 1800 04 05 09
