A clinical psychologist has given evidence in a trial involving the alleged sexual abuse of 13 Northland girls to help a jury understand why children do not report abuse when it happens.

The trial in the Whangarei District Court has been set down for three weeks during which all 13 complainants, who at the time were aged between 6 and 15, are expected to give evidence. James Brian Sanders, 68, has denied 38 charges including rape and indecent assault committed against 13 complainants at Doubtless Bay and Bream Bay between 1998 and 2013.

The alleged offending happened when Sanders was president of the Bream Bay branch of the Latter Day Saints Church and when he helped his wife run an after-school programme in the Far North.

Dr Yvette Ahmad, a registered clinical psychologist with more than 20 years of clinical and academic experience dealing with both adult and child victims of sexual abuse, had not spoken to any of the complainants in the case and had not interviewed Sanders.

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She said there was no set pattern for the reporting of sexual abuse with some children reporting accidentally or on purpose.

"What we do know is children quite often do not tell about the abuse," she said.

Research had shown most people delayed their reporting until they were adults and only a third would report the abuse during childhood. While there might be a delay, it did not affect the validity of the complaint, Dr Ahmad said.

Studies had shown there were a multitude of reasons why children did not raise the issue of sexual abuse when they were young, which included not being believed, they believe nothing would change, a reluctance to burden others and a fear of loss of control of the shared information.

Children who were "groomed" may also delay reporting. Money, lollies and alcohol were given to the child to develop a relationship. Grooming also involves increased intimate touching to gradually sexualised touching with the child. Research showed abusers typically targeted vulnerable children including those in trouble, passive, lacking in self esteem or confidence.

Defence lawyer Chris Muston put several propositions to Dr Ahmad about memory recall and distorted memories.

She said while some peripheral facts may be incorrect the main trauma would be clear and consistent even after a delayed time.