Jason Parkinson, who specialises in compensation claims for institutional abuse victims, is fighting the allegations of professional negligence in a trial in the ACT Supreme Court.

His former client, who cannot be identified, alleges he was repeatedly abused while a student at Marist College in Canberra by the notorious Brother John "Kostka" Chute and teacher Paul Lyons.

He has also accused two other brothers at the college of abuse.

The former Canberra student says he was pressured to accept a settlement offer and then spiraled into depression after signing off on the agreement.

He is suing Mr Parkinson, claiming damages from the loss of chance of a greater payment after a trial and for his psychological injury following the settlement.

Mr Parkinson, the principal of Porters Lawyers, told the court on Wednesday that his firm had dealt with some 40 abuse claims each against Brother Kostka and Mr Lyons.

All those claims were consistent with the "modus operandi" of the two men, he said.

Mr Parkinson said the abuse disclosed by his former client against those two men was "markedly different".

The man had described the Marist brothers forming a circle in the monastery and passing the boy around on their laps to abuse him, the lawyer said.

He had also explained how he was taken to the Sydney Opera House as part of a choir and later taken to an elegant after-party.

During this party, the man had said, he was taken to a darkened room and abused, and he remembered camera flashes and conversations.

"We had never heard [that] at all by anyone ... never subsequently heard of it," Mr Parkinson said of the two allegations.

The lawyer also said the first he heard of the two other brothers the former student had accused of abuse was during this current suit against him.

Of the eventual settlement, Mr Parkinson said: "That was absolutely the best that could be done."

On the advice of Mr Parkinson the man settled his claim for $92,000, for which he would get about $42,000 after the law firm's fees, the court has heard.

In the wake of the royal commission and the revelations about Brother Kostka, in 2017 Mr Parkinson also sought for the man a "top up" of $27,500 from the Marist brothers.

Mr Parkinson's barrister Dominic Villa SC told the court the advice given to the man in March 2010 complied with his obligations to exercise due care and skill and was appropriate in the circumstances.

The barrister said we now lived in a "post-royal commission world".

But at the time in 2010 the man's claim had gone beyond the statute of limitations and it was not likely a court would have given him an extension.

Mr Villa also said even if the man's account was accepted at a trial, there would be difficulty establishing the college was directly or vicariously liable for the conduct.

The trial continues before Justice Michael Elkaim.

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