THE first alleged victim in the historical sexual assault trial against Catholic priest John Patrick Casey has given evidence in a closed session of the Lismore District Court.

Casey has been charged with 27 counts of sexual assault, indecent assault and rape of three boys under the age of 16 said to have occurred in the 1980s.

The trial will cover 18 separate incidents.

Due to a change in law when the majority of the alleged offences occurred (from June 10, 1985 to May 30, 1987) a number of the charges have been paired to address the change in law.

The first alleged victim, now in his 40s, was the third witness to be called to provide evidence in the trial on Tuesday.

The court was closed to the public, which included the media and Casey's 15 supporters, at 12.15pm.

In his opening address on Monday, the crown prosecutor said his case would focus on the testament of three victims to which the charges relate.

He told the jury the first victim would likely talk about an incident that he said occurred at the Mallanganee Presbytery between January 1, 1985 and December 31, 1985.

Defence barrister Charles Waterstreet told the jury in his opening address that Casey would plead not guilty to all the charges.

The first witness to give evidence was the first alleged victim's aunt.

The second witness was the officer in charge of the investigation, a detective senior constable attached to the Casino police station.

A witness who was called to the stand on Thursday gave evidence on the character John Patrick Casey.

He told the court that he and a close childhood friend - the first alleged victim's cousin - stayed with Casey during school holidays in 1986 when he was about 14.

The witness said Casey offered them alcohol two or three times during the stay at dinner.

The officer in charge of the investigation was called back as a witness for video evidence.

The jury was shown footage of the funeral service for the first victim's cousin, where Casey addressed the congregation and spoke fondly of the boys (the witness, the first victim, and the first victim's cousin).

"We had great times," he said in the video.

"I felt like a real father. They were like the sons I never had."

Casey said when the boys left, the house felt empty.

"I questioned the whole thing church has about celibacy. Until I saw a child having a tantrum… then I said, 'thank you for celibacy, Jesus'."

The court was shown two police walk-through videos, filmed last year, of the Mallanganee property where the historical crimes are alleged to have occurred.

At 11am, the court was closed. The trial continues.