At one point Justice Paul Heath told the distressed Ms Montgomerie that she did not have to carry on, but she was determined to tell the court what Boulter had done to her.
She said multiple blows to her head had chipped her skull and at one point Boulter had strangled her and she had lost consciousness.
Outside court, Ms Montgomerie's father Peter struggled to contain his anger and said he had expected it to be much longer.
"It's a f***ing joke, it's a sham.''
The court was earlier told Boulter was out on bail in January last year after being charged with assaulting Ms Montgomerie while the pair lived together in Southland.
Ms Montgomerie left Boulter and moved back to her family on the remote island of Great Barrier in Auckland's Hauraki Gulf.
But the police summary of facts shows Boulter went to great lengths to catch up with her.
He travelled to Auckland from Riverton before booking a one-way ferry ticket to Great Barrier Island using the false name Travis Tetoski.
Once on the island Boulter made his way to Ms Montgomerie's family home where she and her new partner, Michael Stewart, were babysitting her 7-year-old brother and his friend.
Boulter hid under the bed before and waited until the couple went to bed.
He then attacked Mr Stewart and knocked him out. Mr Stewart was left with a fractured skull.
Ms Montgomerie ran screaming from the bedroom but Boulter caught her by her hair and dangled her over the balcony.
He grabbed a weed grubber and threatened her with it before kidnapping her and dragging her across farmland.
Boulter hid Ms Montgomerie in isolated bush and dragged her around the island to avoid the police manhunt.
Boulter also told her he had killed her younger brother and her brother's friend.
At one point he took her behind a barn and strangled her. He told her: "You scream for help and you are dead''.
Ms Montgomerie's ordeal lasted 38 hours before searchers found them.
Crown prosecutor Phil Hamlin said Boulter's offending was made worse because it happened in Ms Montgomerie's home, it involved weapons and Boulter was on bail at the time.
Boulter's lawyer Paul Gruar said his client had lived in 57 foster homes while in the care of Child Youth and Family and had a history of mental illness.
But he said his client's conditions had improved after taking medication in prison while awaiting sentencing.
Justice Heath gave Boulter time off his end sentence for an early guilty plea. He also took into account psychological reports which did not rule out the possibility that the offending was influenced by Boulter's mental illness.
He said Boulter had used "extreme physical violence'' and he could not be sure he had any ability to empathise with his victims.
Justice Heath said he would not impose a minimum non-parole period to give Boulter an incentive to "rid yourself of demons''.
"If you can undergo treatments and programmes and find a way to manage your illness, that will be of benefit to you and the community at large.''
But he said if Boulter did not get help then he may serve the entire eight and a half years. Ultimately, that decision would be up to the Parole Board.
