Shannon Brett Morrison couldn't recall the stabbings but remembered seeing a young man, wearing a t-shirt like that worn by the Boston Marathon bomber, being aggressive towards him, his barrister said on Friday.

Phillip Boulten SC was giving the defence outline at the NSW District Court trial of Morrison, who had worked for an international pharmaceutical company as a research clinical manager for oncology treatment trials.

The 35-year-old has pleaded not guilty to a string of charges including six counts of wounding with intent to cause grievous bodily harm

Prosecutor Fiona Gray said the attacks occurred late one night in January 2019 in a Turramurra park where a group of young people - including the six victims aged between 15 and 19 - were drinking.

Morrison had shared at least three bottles of wine with a former work colleague at a restaurant across the road before the encounter with the teenagers.

They gave accounts of him being angry, pushing and shoving members of the group - some of whom retaliated - and stabbing teenagers in the arms, another in the neck and another in the chest.

"You can imagine the scene, it was chaotic with a number of young people and there was lots of yelling and screaming," Ms Gray said.

Morrison was tackled to the ground and attacked by some of them, before he was arrested.

Mr Boulten said his client did stab the teenagers, although he had no memory of it and only an imperfect memory of events in the park.

"My client was suffering from a very severe mental illness and he was frightened by what he was seeing and what he heard," he said.

Morrison, who had never been in trouble with the police, had a very high pressure job. He also suffered from a well-entrenched paranoid delusional mental illness since he was a teenager.

Because of fears his family would be attacked, he had CCTV installed in their small apartment and began carrying a knife with him.

Mr Boulten said the jury would hear evidence that before Morrison went into the park, another man who was eating pizza was confronted by the teenagers.

He was called "a faggot" and a "pedo" before he was effectively chased out of the park.

Morrison remembered a young man who reminded him of the Boston Marathon bomber pushing him before everything became a blur.

"But you will conclude that the accused took out the knife and he did whatever he did that caused the injuries to the young people who were in the vicinity".

The jury would hear a lot of evidence about who was pushing and who was shoving during the confused incident, in which Morrison was "undoubtedly very very frightened about what was going on".

"His case is he felt trapped, he was outnumbered and he was terrified," Mr Boulten said.

The defence contended he was not guilty of any of the charges because he was not mentally responsible due to his mental illness.

One of the wounded teenagers told police the man "was looking at all the girls, being a weirdo kind of guy hovering" at the top of the stairs, before the man pushed some of them and was pushed in retaliation.

The teen testified to having no recollection of an earlier incident involving a man eating pizza.

The trial continues before Judge Gina O'Rourke.

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Australian Associated Press