She died from suffocation.
Garthwaite was ordered to serve a minimum of 14 years behind bars before he could seek parole.
He has been refused early release multiple times since then.
The killer appeared before the Parole Board again on May 20 and was told he would be freed less than a month later.
Garthwaite will leave prison on Monday - but will be closely monitored.
For the first three months of his release he will be electronically monitored and has a curfew, meaning he has to be at his approved address from 10pm until 5am.
"As to the current position it is clear Mr Garthwaite's rehabilitation is completed," said the Parole Board chairman in a decision released to the Herald.
"As far as reintegration is concerned he has now been on release to work for three years.
"Mr Garthwaite is assessed as being medium-low risk of reoffending.
"He has completed a safety plan, as well as a further period of one-on-one psychological counselling."
Garthwaite also has a number of special conditions he must adhere to for five years after his release, including living at an approved address, not contacting his victim's family, and not travelling to the South Island without express written permission from his probation officer.
Any breaches could see Garthwaite arrested and recalled to serve the rest of his sentence.
In New Zealand, a life sentence is just that - but the Parole Board can release people on conditions.
If those are breached they can be returned to jail.
Garthwaite will also have to reappear before the board by November this year so his progress can be reviewed.
He last appeared before the board in May last year.
Then, Bennett's family told the board they had serious concerns about Garthwaite being released.
"They were concerned he did not demonstrate any remorse for his offending," the Parole Board report stated.
They were also worried that he wanted to be released to a location where the majority of Bennett's family lived.
They told the board that they "continued to suffer as the result of the loss of their mother".
At the time Garthwaite responded that he could understand the victims' very strong feelings towards him.
In prison, the murderer has completed adult sex offender treatment, lived in a self-care unit and participated in the release to work programme.