Police are having to shelve sexual assault complaints because witnesses are too drunk to remember the details.

In Waikato alone, up to five complaints of sexual assaults are recorded each week, usually from women aged between 16 and 30, but many can not be acted on because of the high intoxication of people involved.

The problem also exists at another of the country's busiest police stations, Auckland Central.

Police cannot take a case to court without clear evidence.


But some complainants were so drunk that officers' investigations - costly in police time and resources - were left in in limbo.

"Sometimes it is so bad that they can't remember anything, nothing at all about the sexual assault," Detective Senior Sergeant Mark Greene of Hamilton told the Weekend Herald.

"But they know they have been assaulted. It's also rare that someone walks in here and it's not genuine."

Detective Inspector Scott Beard of Auckland City CIB said the situation had long been an issue for police trying to investigate complaints.

"It's unfortunate that, when people do overindulge, they can become more vulnerable to predacious opportunists who may have undesirable, misguided or criminal intent towards them.

"People who are highly intoxicated are also impaired when it comes to recall of specific events and, often, their evidence - if ever put before a court - can be deemed unreliable or questionable."

Of the 3448 sexual assault and related offences recorded by police throughout New Zealand in the most recent year for which statistic have been compiled, 42.5 per cent were not resolved.

Police in the Auckland central, east and west districts resolved 160 of 257 offences recorded, and Waikato police resolved 208 out of 362.

A Massey University survey released this week showed 28 per cent of 16 and 17 year old girls were binge-drinking, downing at least eight standard drinks in a typical drinking session.

The proportion of binge-drinking young women had doubled in less than a decade.

And emergency departments throughout the country were treating more women for intoxication than ever before.

Mr Greene said binge-drinking played a big part in the problems police faced with sexual assault complaints.

"It's definitely part of the fabric of it."

In Auckland, Hamilton and other centres, police and other agencies are out in the streets at night, pushing campaigns designed to prevent sexual assault.

Hamilton police are giving the message, among others, that intoxicated or inebriated people who are slurring their words or stumbling around can't properly consent to having sex.

"You get a lot of young women who go to parties or whatever beforehand and when they get to town they are absolutely written-off - we are seeing a lot of it," said Detective Nicola Cornes, one of the officers taking the campaign to Hamilton bars.

"I think a lot of them think they are prepared and safety-conscious but they are probably not."

Police have also increased their training, with a focus on adult sexual assault offending during CIB induction courses.

"We are trying to ensure our staff have a far more comprehensive understanding of the dynamics of this type of investigation than they had before," said Detective Inspector Tusha Penny, manager of the national child protection and sexual violence team.

While cases involving intoxication could make investigations challenging, they were also an impetus for police to get out and explore other avenues for evidence.

"We make sure we do a thorough investigation and at end of the day we would hope we have established the truth of exactly what happened," Ms Penny said.

There were large concerns that sexual assault remained significantly under-reported and research was being done to analyse why many victims did not make complaints.

"We want people to tell us about it, because people need support ... and we need to make sure people doing that stuff are being held to account."

Rape Prevention Education director Dr Kim McGregor believed much of the solution lay in prevention.

She said victims were too often blamed because they had been drinking or wearing revealing clothing.

"Nobody goes out thinking they better not wear a short skirt because they might be raped - so let's start focussing on the offender and how we can intervene in their behaviour, rather than blaming victims."