THE trial of Catholic priest John Patrick Casey over allegations of historical child sexual abuse and rape has begun in the Lismore District Court.

Casey - who was involved with the Mary Help of Christians Primary School in Sawtell until the day prior to his arrest - has pleaded not guilty to all 27 charges relating to 18 individual alleged events.

The charges include sexual assault, act of indecency on a person under 16 years of age and homosexual intercourse with a male under 10.

In delivering his opening address, the Crown prosecutor said the cornerstone of the case would be the testaments of three victims, who were all aged under 16 when they said the sexual assaults and rapes occurred in the 1980s.

The Crown prosecutor said the first victim, now in his 40s, would likely tell the court about a sexual assault in the shower at the Mallanganee presbytery, west of Casino, in 1985.

He said the victim, then aged 12 or 13, would recount going into the bathroom to have a shower and having the accused follow him in, undress, get in the shower and offer to "wash him".

The prosecutor said the victim planned to tell the court the accused started washing his chest before moving down to his genitals for an "excessive period of time" and saying, "got to make sure it's clean".

The remaining charges relate to two brothers, who were both under the age of 13 when the alleged events occurred at Mallanganee between June 10, 1985 and May 30, 1987.

Seven of the charges related to the older brother, now 42, who was between the ages of 11 and 13 when the offences are alleged to have occurred.

The prosecutor said on one occasion, when the brothers were in the care of Casey and staying at his home, the second victim asked the accused about a fluffy toy seal.

He was taken from the pool to a shed in the backyard where toys were stored.

He said when the second victim reached up to grab the toy, the accused put his hands down the front of the boy's swimmers and said, "this is between us, you can't tell your mum and dad". The incident would become known as "the first seal incident".

The remaining 19 charges relate to the younger brother, who was between nine and 10 when the events allegedly occurred. In one event, the prosecutor told the court, the accused was naked in the shower with the third victim when he took the child's hand, put it on his penis and said, "it's okay to be nervous".

The court was told another charge related to allegations Casey raped the young boy on a bed in the presbytery.

Prominent defence barrister Charles Waterstreet said his client would plead not guilty to all the charges.

"Every disgusting act, he did not do," he told the court.

Mr Waterstreet said the defence case was that "nothing of a sexual nature took place".

"These boys then, men now, have so interrelated with life's circumstances... to end up in this court 30 years later," he said.

"(The court) might bear in mind these many years ago (there was) one outstanding missing link. No one was there. No one saw them but the complainant and the accused."

Mr Waterstreet said the court would likely hear one victim had been assaulted by an older brother, which had exposed him to trauma where "he even believes what he says, 30 years later, and it's just not true". He said defence would also submit "family dynamics".

"Nothing in family is simple. Nothing in life is simple. People can lie," he said.

The trial is estimated to take between three and four weeks.