What's one way to prevent sexual assaults on a boozy Friday night in the city?

"Embarrass the kids."

Detective Sergeant Grant Carroll has no qualms walking up to a "canoodling" couple in an alleyway around Wellington's Courtney Place and asking blunt questions.

"How do you know she's consenting?" He'll ask.

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Most of them can't answer. "They're too plastered."

But an intrusion from the police can be what it takes to break the spell for an intoxicated potential victim who doesn't actually want to be there.

The embarrassment is worth it to prevent a sexual assault report rolling in a couple of days later.

With two or three adult sexual assault (ASA) reports being made after each weekend, directly related to Friday and Saturday nights in the inner city, Wellington police are working hard on prevention, and it includes looking out for "predatory behaviour".

In a briefing to officers ahead of their Friday night patrols, Carroll reminds them of some examples to keep an eye out for.

"Look for guys walking with their arms around the girls, trying to separate them," he said.

"Keep looking for that behaviour that could lead on to more serious offending."

Arm placement can be important – Carroll watches out for men walking with their arm around a woman's head or neck, with their hand on the back of the head. It can be a sign of controlling behaviour.

"So you want to get in there and make sure that that girl wants to go with that guy . . . we do it in a friendly way."

He watches out for signs women are being separated from the rest of their friends.

"Go out as a group of five, come home as a group of five," he said.

"Because we work in here we can see the damage a sexual assault can have on someone's life and the ability to get on with their lives . . . the penalty for getting consent wrong is huge," he told the Herald.

"We're pushing the 'don't guess the yes' thing.

"The Wellington police have done a really big push on the education side."

They work with universities, liquor licence holders and other groups to teach people about consent, and to put prevention measures in place.

"It's one question I always ask the male offenders. How did you know your partner consented?"

Police work on being visible in the city at night, being approachable, and taking every opportunity to prevent harm.

"We just want everyone to have a good night and get home safely."

There are 12 dedicated staff working in the Wellington ASA teams.

Inspector Wade Jennings said there had been a significant drop off in "serious stranger-type incidents", which were now rare in the CBD. Most incidents involve a person known to the victim.

"While we believe people are also a lot more confident to report incidents to the police we recognise that sexual assault is hugely under reported to police and often it is many years after a sexual assault before a victim [or] survivor decides to or is strong enough to report a sexual assault to police," he said.

"It is never too late to report this to police. Police have specially trained investigators who will listen to victims [or] survivors and talk them through the investigation process."

Over summer an extra four people will be assigned to an ASA prevention team, working closely with partners such as universities and schools.

Detective Sergeant Ben Quinn, of the Wellington ASA team, works alongside the Sexual Abuse Prevention Network, the Wellington City Council, and representatives from the Hospitality Association of NZ, on the possibility of training bar staff in being aware of, preventing, and dealing with reports of sexual offending at the city's nightspots.

Potential "formal training" includes what to do before, during, and after incidents of sexual harassment and assault, and techniques for security and bar staff when dealing with a potential offender or victim.

"It also includes considering what bars and other premises can do to improve on the safety of their environments for patrons," he said.

"Anything which helps people identify and support those who are potentially at risk of being a victim of sexual offending is an extremely positive initiative, and something police is keen to support as a prevention tool."