In an open letter, delivered on the eve of a WA trial, the Christian Brothers have admitted several brothers sexually abused child migrant John Thomas Lawrence for years from the age of nine in two Perth boys' homes.

Key points: Mr Lawrence says he was sexually abused by four people while in two orphanages

Mr Lawrence says he was sexually abused by four people while in two orphanages The abusers included three Christian Brothers and a lay teacher

The abusers included three Christian Brothers and a lay teacher Mr Lawrence says he was left suicidal and is seeking compensation

WARNING: This story contains material that some readers may find upsetting

But a lawyer for the organisation has argued he deserves a lower compensation payout than what his lawyers have sought because his poor upbringing meant he had a low earning capacity, regardless of the abuse.

The case is the first of its kind to go to trial since Western Australia removed a time limit restricting when child sexual abuse cases could be brought.

A similar case brought by a terminally ill man, Paul Bradshaw, was settled for $1 million before proceeding to trial in 2018.

Paul Bradshaw received a $1 million settlement after he was abused by the Christian Brothers. ( ABC News: David Weber )

Mr Lawrence's trial was due to begin today, but his barrister, Tim Hammond, revealed the Christian Brothers had admitted "many facts" about the abuse at the Clontarf and Castledare orphanages in an open letter delivered on Monday night.

The facts Mr Hammond detailed included an incident at Castledare Boys' Home, which began when Mr Lawrence wet the bed.

Mr Hammond told the Perth District Court Mr Lawrence was nine at the time and his abuser, now deceased brother Lawrence Murphy, was in his mid-30s.

The Castledare Boys' Home was run by the Christian Brothers in Perth. ( Wikimedia: Moondyne )

The court heard after Mr Lawrence wet the bed, Brother Murphy ordered him to take off his clothes.

Mr Hammond said Brother Murphy then raped him as Mr Lawrence cried and screamed.

It was the first of six rapes Brother Murphy perpetrated against Mr Lawrence.

Molestation by multiple people

But Mr Hammond said the abuse was not contained to one offender.

He said another brother, Francis Marques, who is also dead, molested Mr Lawrence while they watched movies.

In another case of abuse, a third now-deceased brother, Alonzo Angus, fondled his genitals.

In a fourth case, a lay teacher, Joey Jackson, who has also since died, made Mr Lawrence wear lipstick and a dress while he abused him.

Many instances of abuse against Mr Lawrence have been admitted to by the Christian Brothers. ( ABC News )

Mr Hammond said Mr Lawrence, who is now 75, had attempted suicide many times throughout his life and struggled to hold down a job.

Losses estimated in the hundreds of thousands

Mr Lawrence's legal team estimated lost earnings to be worth between $715,000 and $850,000, with an estimated $48,000–$58,000 submitted regarding lost superannuation.

Bettina Mangan, the barrister representing the Christian Brothers, has indicated she would argue for a lower amount.

Ms Mangan told the court Mr Lawrence would have had lower wage-earning capacity because of his poor upbringing, regardless of whether he was abused.

"It seems illogical and unfair," she conceded under questioning from Judge Mark Herron.

"Brutal," he responded.

"The court can't escape from the fact he didn't come from well-off, middle-class [parents]," Ms Mangan said.

A sum of $111,000 already paid to Mr Lawrence by the Christian Brothers for his abuse would be deducted from any compensation payment determined by the court.

The trial continues.