A "possessed" woman who collided with a motorcyclist on the New South Wales north coast has been found not guilty of murder due to mental illness.

The defence did not dispute that 49-year old Vanessa Fraser killed father-of-three Trevor Moran on the Pacific Motorway near Cudgera Creek, north of Byron Bay, in January last year.

The court heard she had been suffering psychotic delusions while driving at 180 kilometres per hour.

Ms Fraser told police she was feeling "possessed" in the lead-up to the incident, and thought she could save the soul of Scottish actor Ewan McGregor by driving fast towards the Tweed Heads Bridge.

The court also heard Ms Fraser had been admitted to mental health units in Tweed Heads and Lismore on several occasions in the months leading up to the tragedy.

Each time her condition improved under medication, but she refused to keep taking it once released, and smoked 1 or 2 grams of cannabis each day, the court heard.

Supreme Court judge Desmond Fagan told the court the difficulties in managing mental illness in the community were enormous.

"Involuntary confinement should only be used cautiously and with restraint," he said.

"But this case shows a patient experiencing episodic psychosis may not present a high risk to public safety, yet may commit a lethal act behind the wheel of a car."

Accused apologises in court

Ms Fraser tended a letter of apology to the court.

It said in part: "Mental illness struck me late in life, at the age of 42, so I always had trouble accepting the diagnosis, instead thinking that my mind would go back to being normal," she wrote.

"Since the accident I now know that I will require medication for the rest of my life to ensure that nothing that can cause harm to others can ever happen again."

Detective Senior Constable Scott Wilcox told the ABC the initial investigation of the case was made more difficult because no-one saw the actual crash.

"No-one actually saw the point of impact, which for me was surprising because there were so many people travelling on that road on that day," he said.

"Sometimes collisions occur because of inattention or there's a drug or alcohol problem … where in this case it was a disease of the mind which has ultimately caused the collision."

Senior Constable Wilcox said mental health cases were some of the more difficult aspects of policing.

"If you think somebody shouldn't be in a car or if they're going through a tough time, if they're displaying any signs of psychosis or depression or anxiety or anything, I think what every one of us can do at a minimum is ask them if they're all right," he said.

"I think doing something is always better than doing nothing, even if you think it's something very minor — who knows, you might just save something like this happening again."