McCormack had been using LSD with friends and had what she described as a "panic attack", believing her heart was going to stop and that she needed to get to hospital.
She dashed through traffic and pulled open the driver's door.
"Get out of the car. Give me the car," McCormack screamed.
She continued to yell her demands as she repeatedly punched the man in the head.
When the driver did not vacate his seat, McCormack crawled over him, "squashing" the seated nun, and kicking at the driver while flailing her arms.
The driver managed to get out of the vehicle before pulling McCormack out by the ankles.
Members of the public restrained her on the footpath while the victim locked his car.
But McCormack broke free and again tried to access the vehicle before her eventual arrest.
The driver suffered cuts and bruises, while the nun sustained a cracked rib.
Judge Michael Crosbie said the level of violence meted out was "staggering".
"This episode was simply awful — elderly people out, minding their own business," he said.
McCormack's behaviour caused some concern while behind bars, the Parole Board heard at a previous hearing.
She was described as "aggressive" and had been tattooed by another prisoner, the board heard.
Since then, panel convener Judge Phil Gittos said, reports on her conduct had been "entirely satisfactory".
McCormack, who had 25 convictions for dishonesty, burglary and violence, was deemed unsuitable for group programmes but had completed one-on-one counselling.
"We are satisfied that Ms McCormack may now be released without undue risk to the safety of the community," Judge Gittos said.
Her parole conditions include:
• To live at an approved address in Christchurch.
• To comply with the tenancy rules of the accommodation.
• To abide by a 10pm-6am curfew for the first three months.
• To attend assessments and any treatment prescribed by Probation.
• Not to possess alcohol or non-prescription drugs.
• To disclose to Probation details of intimate relationships.
• Not to contact victims.
