She pushed him into a Christmas tree then slapped him in the face before blocking his exit.
When the victim tried to leave, Cairns threw a barrage of punches, then kicked him in the groin as he tried to restrain her.
While on bail, her violent side became apparent again.
The pair were in central Dunedin on August 1 last year when the intoxicated defendant became upset and left her sober boyfriend at a bar.
When he pursued her, she hit him in the face then pushed his head against a wall.
Later, when the couple were in the car, Cairns punched the man four times in the right eye, leaving him in tears, the court heard.
"She would wait for him to lower his guard then strike him," a police summary said.
Cairns calmed temporarily but became enraged again when she discovered she had lost her watch during the assaults.
The defendant reached speeds of 130km/h in a 50km/h zone, threatening to crash the car.
The victim averted tragedy by applying the handbrake then demanded Cairns allow him to take over driving.
But while he drove, the onslaught continued.
Cairns hit the man eight times in the head and body, for which he later sought medical attention.
The court heard the defendant attacked her ex-flatmate a month later, punching and slapping the victim up to 25 times.
"At 20 years of age, I have to say I'm troubled by where you're heading," Judge Spear said.
Counsel Andy Belcher said his client was supported by her employer, who would continue to offer her work despite knowing the details of her offending.
Cairns was sentenced to four months' community detention and 12 months' intensive supervision and disqualified from driving for six months.
