A "one size fits all" approach to cross-examining child sex abuse victims can make their evidence seem unreliable in criminal trials, an inquiry has been told.
The director of the Tasmanian Law Reform Institute, Terese Henning, told the sex abuse royal commission on Wednesday legal requirements of the rules of evidence could impose barriers for child witnesses, including those with disabilities, and mean they are stereotyped as unreliable.
Children can be worn down by cross-examination and might agree with questions when they don't understand them, Ms Henning said.
"So the nature of the questioning may produce unreliable evidence from them when they are in fact reliable witnesses," she said.
Ms Henning said the problem was often the style of questions and legal conventions around cross-examination.
The royal commission has been looking at criminal justice issues that arise in prosecutions of child sex offenders.
The two-part inquiry was told last week that a young man who was abused when he was 12 by a teacher at a private school in Perth was asked if he had led the teacher on.
Other witnesses, including the mothers of very young and disabled children, have this week told the commission they no longer had faith in the criminal justice system, which failed to prosecute pedophiles because it was decided their children were unreliable witnesses.
The commission is exploring whether reforms in the adversarial trial system are needed to serve vulnerable people in child sexual abuse cases.
It has been looking at how the requirements of the criminal justice system relating to oral evidence and cross-examination work for young children, or those with disabilities and communication difficulties.
Ms Henning is one of a number of experts who will give evidence to the commission.
Earlier on Wednesday, the commission heard that courts and juries needed expert help to understand how very young children remembered and responded to sexual abuse.
There are provisions for expert evidence in the rules of evidence but a NSW Crown prosecutor, Huw Baker, said in his experience they were not called on to help prove a child had been abused.
Mr Baker in 2012 decided not to proceed with the prosecution of a man charged with the sexual abuse of three toddlers at a south Sydney childcare centre, because he deemed conviction was unlikely based on the evidence of the children.
On Wednesday he said that the case could have benefited from expert witnesses who could explain how children who have been abused might think and behave.
The commission has heard that a child may not remember specific detail but have composite memories of their abuse, or counter-intuitively may still relate well to their abuser.
In the current system, this can be presented to juries as evidence the abuse did not happen or as unreliable evidence.
