A jury has ruled a man was mentally ill when he attacked a group of teenagers with a knife in a Sydney park on January 12 2019.

University of NSW cancer researcher Shannon Brett Morrison, 35, went on trial in the District Court for the attack which wounded several teenage girls and boys, aged 15 to 19.

He pleaded not guilty to six counts of wounding with intent to cause grievous bodily harm, one of assault occasioning actual bodily harm and one charge of possessing a knife. He spent six months in custody after the stabbing in Cameron Park in January 2019.

The court heard that Morrison was extremely paranoid, installing security cameras at his home and slept with a knife by his bed in the days before the stabbing rampage.

Mr Morrison had been drinking for hours with a date at a bar near Cameron Park, before leaving around 11.30pm. He went into the park after telling his date he needed to use the bathroom, before producing a flick knife when confronted with a group of teenagers.

Six teens were stabbed. A teenage boy was slashed in the neck during the incident.

His defence lawyer argued Morrison was suffering from psychotic mental illness at the time of the incident, which may have been triggered by something in the park.