Just 12 per cent of reported rapes result in conviction, according to a Law Commission study.
Wellington Rape Crisis manager Natalie Gousmett cited the figures in a Law Commission review which the organisation wanted to see reinstated by the Minister of Justice.
People who were sexually assaulted were the least likely of all victims of crime to report to the police, the review found.
"An estimated 90 per cent of sexual offences go unreported," the review said.
"After allowing for a number of withdrawn cases, a United Kingdom study found that in 2003/4, 21 per cent of reported rape offences went to court and 12 per cent of these resulted in a conviction.
"The conviction rate for sexual offences is also lower than for other crimes."
Ms Gousmett said a Wellington Rape Crisis social worker told her that in the last year she had worked with only five cases in which the rape was reported to police.
"One of them went to court and one resulted in a conviction."
There were numerous reasons for low conviction rates, the Law Commission review said.
"In many sexual violence cases there are no witnesses, little or no physical evidence, and the issue is whether the complainant consented and if the accused had reasonable grounds to believe the complainant consented.
"The fact that sexual violence is more likely to be committed by a person known to the victim can further add to the complexity and difficulty in successfully prosecuting this crime."
Crown Prosecutor and Law Society spokesman Jonathan Krebs said that when deciding whether to pursue a charge of rape through the courts, police would first take a statement from the purported victim.
"If it's something that's very recently occurred and there's any chance of getting there being any forensic evidence, then tests are usually undertaken.
"Enquiries are then usually made by police with any witnesses who might be able to corroborate the complaint and then at that point an assessment is made whether there is enough evidence to justify a conclusion that an offence has occurred, and then [they] bring a suspect.
"Then a decision has to be made as to whether a charge ought to be laid."
Generally, the sufficiency of the evidence was taken into account, Mr Krebs said.
Police had to make an assessment of the veracity of the complaint, which was needed from a societal point of view, he said.
"It's important that things are not taken at face-value, otherwise complaints would be themselves a weapon to be used by nefarious people."