Child sexual abuse within the Australian Defence Force over more than five decades is being examined by the royal commission at hearings that started in Sydney today.

The latest royal commission case study is focusing on the experiences of survivors of child sexual abuse at HMAS Leeuwin in the period 1960 to 1980, the Army Apprentice School Balcombe in the period 1970 to 1980 and ADF Cadets in the period 2000 to present.

The inquiry will also look at the "systems, policies, practices and procedures of the ADF and the ADF Cadets to prevent child sexual abuse and dealing and responding to concerns and complaints about child sexual abuse" according to the royal commission's statement.

Thirty witnesses are expected to give evidence at the Royal Commission into Institutional Responses to Child Sexual Abuse before commissioners Justice Peter McClellan, Robert Fitzgerald and Professor Helen Milroy.

Adair Donaldson, a lawyer acting for 50 people who suffered sexual abuse while at the ADF when they were minors, said: "What we are going to hear is a number of courageous survivors who are prepared to tell their stories about the horror they experienced."

"By telling their stories, they hope that they will make a difference for the future and that what they experienced will never be experienced by any other member of the ADF," Mr Donaldson said.

He told the ABC the royal commission would expose the "systemic failures that allowed the abuse to occur".

"But just as significantly I hope we're going to hear how the ADF has changed."

'The abuse that our clients suffered was horrific'

Lisa Flynn, whose legal firm acts for two people who are giving evidence before the royal commission, said they were 16-year-old sailors at HMAS Leeuwin in the late 1960s.

"The abuse that our clients suffered was horrific, including physical and sexual abuse which had a significant impact on their lives," Ms Finn said. "There was an accepted culture of bullying, physical and sexual abuse. "Often incidents were reported but because of the culture nothing was done to stop or prevent further abuse from happening."

Ms Flynn said the ADF had recently made some "positive progress in the restorative justice outcomes provided to survivors".

A Department of Defence spokesman said Defence was cooperating with the royal commission and supports its objectives to safeguard children.

"Understanding the nature and extent of historical abuse is fundamental to improving our culture and how our people are treated," the spokesman said.

"We recognise and commend the courage of those who will tell their own stories of personal suffering throughout the hearings."

Child sexual abuse at ADF 'systemic'

Barry Heffernan is a Vietnam veteran who provides support to victims of abuse at the William Kibby VC Men's Shed at Glenelg North in Adelaide.

"The thing we do is we listen, and we take calls at 1,2,3 o'clock in the morning, people who are just crying their eyes out because they're having nightmares and all that sort of stuff," he said.

"The big problem is, a lot of people don't report it."

Barry Heffernan says many victims have not reported the abuse. ( Supplied )

Mr Heffernan said over the past five years he had been approached by more than 400 veterans who were abused during their time in the Australian Defence Force.

Many of them were children.

"It was systemic. These were children, who'd left home, no mother to go and talk to or anything else like that.

"It was just a sad time, where nobody had any security or support or anything.

"Christ knows how many people attempted suicide, and some succeeded."

He said many victims were cautious about reporting their abuse to the ADF.

"People have a problem going to Defence," he said.

"A: because they don't trust Defence investigating Defence, and B: the question from male and female, mostly females was 'how do I know the bloke I'm reporting my incident to isn't the best mate of the guy who just raped me'?"