Belmont twins Stephen and Richard Mateer in court over historical sex assault charges

Stephen Mateer is accused of 45 counts of sexual assault, while his twin brother Richard Mateer is accused of ten counts against the same two complainants.

Justice Peter McGrath has accepted tendency evidence revealing each of the two men were known to be sexually interested in young boys and had a tendency of acting on it.

Stephen Mateer allegedly raped and sexually touched boys he met through a Lake Macquarie pizza shop while they were aged between 12 and 15 years old - from 1989 to 1993, the court has heard.

Stephen Mateer, 70, was earlier deemed unfit to stand trial due to impaired cognitive ability believed to be dementia.

He has not entered pleas and is facing a special hearing under mental health legislation in Newcastle District Court, which started in February and resumed this week.

That hearing has run concurrently with the judge-alone trial of Richard Mateer, charged with one count of rape and six charges of assaulting and committing an act of indecency against one of the alleged victims - a 13-year-old - in the company of his brother. He has pleaded not guilty.

Victims of previous offending were similar ages to the complainants, the offending was similar to that alleged, and all of it occurred in the same area.

Stephen and Richard Mateer were arrested after detectives from Strike Force Arapaima - investigating the long-unsolved disappearances of Lake Macquarie girls Robyn Hickie and Amanda Robinson - were tipped off to a string of alleged sexual assaults against two boys in the late 1980s and early 1990s.

It is alleged that the attack involving both Mateers took place in the back of a van.

However, Richard Mateer has vehemently denied ever offending or engaging in any sexual activity in the company of his brother.

In the twins' defence, Justice McGrath is told that some of the complainants' evidence casts doubt across the credit of the other due to inaccuracies and inconsistency about who did what, to who, and who was present at the time.

Identity is a critical issue, defence barrister Stephen Ryan said on behalf of his client Stephen Mateer.

Another inconsistency was evidence from one of the complainants who said he tried to talk to the other one about the alleged assaults when they were 17 or 18 years old.

One of them had already left the area by then, and the other one had no memory of the conversation, which was difficult to accept - two youths, in their late teens, trying to broach a very difficult subject, Mr Ryan said.

"They did their best to recall certain things but with the passage of time and the lives that they have led, it's the accuracy as to whether they have got the right person or people and whether the acts they allege to occur are made out," Mr Ryan said.

"This would be the first time based, on the tendency material ... the twins would have offended together which is an odd point in this particular trial."

The matter is back before Justice McGrath on Friday, May 26.

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