Corrections says it is powerless to stop prisoners making phone calls to their victims if they have help on the outside.
The admission came as a convicted rapist was sentenced for phoning his victim from prison while posing as a doctor and trying to influence her evidence.
Pravin Fia Hari Prasad Kumar was already serving an indefinite sentence of preventive detention after failing to turn up to his own trial where he was convicted of rape, indecent assault and kidnapping.
He was sentenced in the High Court at Auckland today to a further three years after being found guilty of perverting the course of justice.
The 43-year-old was acquitted of a second charge after a two-day hearing before Justice Geoffrey Venning.
He has appealed against his conviction.
The Department of Corrections' northern manager Jeanette Burns said prisoners could only call people on a pre-approved list. Their conversations were recorded and randomly monitored.
But Kumar was able to contact his victim by phoning his mother who diverted his calls.
"This is disturbing and remains a problematic issue for Corrections," Ms Burns said. "Prohibiting prisoners from making calls at all would be impractical and illegal."
Wellington Rape Crisis manager Natalie Gousmett described the case as "absolutely horrific" and said more needed to be done to protect victims.
That sentiment was echoed by Rape Prevention Education director Dr Kim McGregor, who suggested some kind of technology was needed to stop prisoners' calls from being transferred by third parties.
She said the woman had already been through the trial and would have believed she was safe from Kumar.
"To finally put that all behind you and to still not be safe from the offender would be even more disturbing."
The mother of the victim said she had raised concerns with the appropriate authorities but did not want to comment further.
This isn't the first time a prisoner has been convicted of harassing someone on the outside.
In 2009 Elton John Taniora-Waitai received extra jail time after stalking a mother of three he had seen on a TV documentary.
He pleaded guilty to two charges of impersonating police and one of criminal harassment after tricking police into handing over the woman's details.
At the time Corrections told media they had tightened procedures.
Today the High Court heard Kumar was able to reach his victim through his mother, who transferred his call to the mental health institution where Kumar's victim was staying in June last year. It it thought she has not been charged.
Kumar posed as the psychiatrist who examined the woman and spoke to her caregivers before talking to her and pretending to be another doctor.
He chatted to her about her medication and her cat before directly challenging her evidence.
At one point Kumar suggested she had only been touched. The woman replied that she had been "properly raped".
Crown prosecutor Alysha McClintock pointed out the vulnerability of the victim and said the calls would have continued had police not intervened.
Kumar's lawyer Simon Lance told court his client had not threatened the woman.
He said Kumar had been frustrated with the legal system after being convicted for rape in his absence.
Justice Venning said that was because Kumar had refused to show up: "He created the problem."
Justice Venning said Kumar's victim had suffered a relapse of her mental health condition and self-harmed when she discovered it had been Kumar who contacted her.
He said Kumar had been trying to get the woman to withdraw her evidence or change it.
Justice Venning described Kumar as having an "impoverished motivation for change" and a lack of insight into his offending.
"Your conduct was persistent and your attempt to influence her was insidious."
Kumar had to be forcefully removed from the dock at the end of the sentencing as he tried to protest his innocence.