At his house, Ashby verbally, physically and sexually assaulted and threatened the woman with a beer bottle, pool cue and screw driver before she escaped.
During the ordeal Ashby had the keys to her car and would only allow her out if he was with her. On one occasion they drove to Moerewa to get food.
People visited the house while she was there but she was too scared, fearing for her life, to raise any alarms.
Ashby committed the offences while serving a two-year sentence of intensive supervision, imposed in July 2009, on a charge of violence against a woman.
In court yesterday, Crown prosecutor Anna Patterson said the victim was confused, manipulated and did not have a lot of self empowerment while being kept captive.
A starting point of between 14-16 years' jail was appropriate, Ms Patterson submitted.
The Crown asked the court to impose a minimum non parole period.
Defence lawyer Grant Anson said Ashby had the ability to respond to rehabilitation and that his difficult start to life must be taken as a mitigating factor.
Justice Brewer said the victim felt numb, fearful, isolated and terrified and had nightmares as a result of Ashby's actions.
Ashby had a difficult childhood, moving between different whanau residences.
He went into the care of Child, Youth and Family (CYFs) when he was 11, the judge said.
In the six years before his latest offending, Justice Brewer said Ashby amassed 19 convictions for violence, including assaults on police and prison officers.
He was assessed as having a low intellectual capability, poor impulse control and a lack of internal control.
Justice Brewer imposed consecutive terms of 15 years' jail on sex charges, six years' for kidnap, 18 months' for assault with a weapon and one year on male assaults female charges.