Data released to the Herald under the Official Information Act shows there were 2089 reports of sexual assault of victims in that age bracket last year. The equivalent of more than five each day.
Manager of the adult sexual assault and child protection team Detective Inspector Dave Kirby said all sex offences across the country had increased by about 17 per cent since 2016.
Data shows reported assaults on 12 to 18-year-olds is up 6.7 per cent in the last three years. There were 1957 reports in 2017; 2053 in 2018; and 2035 in 2019.
With 1695 reports in the first eight months of this year alone, the trend looks set to continue.
The Counties Manukau policing district had the highest number of reported victimisations for sexual assault among youth with 256. This was closely followed by Canterbury with 252.
These areas do have the highest populations with approximately 642,000 and 617,000 respectively.
Last year, 44 per cent of all sexual assault reports were from people aged 19 and under, Kirby said.
Those aged 29 and under made up 67 per cent and 34 and under made up 74 per cent.
He said the increase is not concerning and is due to more people reporting crime.
"An increase, for us, is usually really good. We don't perceive it as an increase in offending."
Often a large proportion of offenders in sexual assault cases involving a person under the age of 18 are known to the victim, Kirby said.
"There is plenty of data that shows a higher proportion of sexual offenders are known by the victim than not.
"It depends on the ages but younger ages, around the 14-year-old age bracket, most of them are known."
Historically there has been a huge amount of under-reporting in sexual assault cases and a societal change is needed, he said.
"There are so many factors why children and young people don't report. Fear, intimidation. Fear of not being believed is a big one too at that age.
"We need to believe our children when they talk about it."
He said often it shows in behaviour changes, which can be an indicator parents might need to step in.
"Our staff around the country really want to hear from anyone who has been subject to sexual assault. We've taken a great deal of time to train staff to be able to respond with empathy and support."
"I felt weak. I felt used."
Sally* told the Herald she was sexually assaulted while attending a party in Year 11.
"We had all consumed alcohol and by the early hours of the morning the party died down and a group of people including myself crashed at the house."
Sally said she found a spot on the floor of the lounge to sleep but when she couldn't, she went to the bathroom.
She did not lock it and within seconds a male aged the same as her walked in, shutting the door behind him.
"I felt scared, and at the time had not experienced any sexual encounters other than kissing boys, and was uninterested in doing so."
She eventually managed to push him away and went back to the spot she had been laying in. The boy then lay beside her, grabbing her hands and putting them on his erect penis.
"He said I was playing hard to get and it was exciting him. I freaked out and told him no several times before he persisted with my hands underneath his in motion of a handjob.
"He was strong, I was scared. There was also the factor of me not knowing who I was surrounded by in the lounge that made me scared to yell out to anyone and wake them for help."
Sally said she tried to get up but he pulled her back to the floor, putting his hands down her pants.
"This was the first time I was touched there, ever. It hurt, I bled during and afterwards for days, and I hysterically cried."
The boy continued to indecently touch her, Sally said, and she tried waking up the boy beside her with her free hand.
"He rolled over and did nothing about it."
The ordeal only ended when the sun came up and people could be heard talking in the other room.
"Once I had got home he had found me on social media and messaged me saying he was still drunk when he got home and he was sorry if he did anything bad. He knew. But played it down."
The boy she had tried to wake up also messaged her asking if she was okay and explaining he felt helpless and was ashamed of himself for not helping.
"I felt completely and utterly disgusted in myself and how I handled that situation. I felt so much fear for what could happen if I didn't give him what he wanted.
"I felt weak. I felt used. I felt like I now had a sticker on my head saying, "I'm easy."
She did not tell anyone what had happened until a couple of weeks after when she had a "breakdown" to her mother.
She said every female she knows has experienced "slight" maltreatment by men in the sense of sexual remarks and inappropriate touching.
Victim support advocate Ruth Money told the Herald although there has been an increase, most sexual harm offences still go unreported.
"I think there have definitely been improvements in certain areas of the system for survivors of sexual abuse. It's not as re-traumatising or abusive as it was but we could certainly go a long, long way further to improve the experience," she said.
A big part of the solution is more education around sexual harm in the community, Money said.
"There are so many young people I work with who don't realise what they've been exposed to is sexual violence. Many people think because they didn't say no, they weren't raped and that is not the case at all."
In the 12 to 18-years-old age bracket, there is a lot of alcohol involved in reports of sexual harm, she said.
"Society puts the shame on the survivor, you know, she was drunk therefore she deserved it. That then leaves them to be really cautious about reporting anything because they believe because they were under the influence it was their fault."
Another key issue for young people is the increasing use of pornography, she said.
"Things like 9-year-old boys watching porn on school grounds because they can, it's free.
"Don't bury your head as a parent, talk to them about porn. What they see on that screen becomes normalised and their first sexual encounters are strangling or slapping and it's just not real. It's not the normal world."
Money agrees that parents need to talk to their children about sexual harm and be on the lookout for signs.
"My very young survivors will go mute. The teenagers, school will drop off for them, they can't concentrate anymore."
Christchurch Girls' High Survey
The data follows the release of a survey at Christchurch Girls' High School in August that showed more than a quarter of students said they've been sexually harassed more than 10 times - and more than 20 students said they have been raped.
The school commissioned the survey after concerns were raised around the rise in harassment towards students.
Three formal reports were made to police by students in the days following the survey.*
CGHS survey principal Christine O'Neill said they do not intend to run the survey again next year as it was a significant undertaking "but well worth it".
She said they have recruited a third counsellor and continue to work closely with police as needed.
"We would still like to see a national uptake on this to have a picture across secondary education."
Researcher Liz Gordon, who undertook the survey work, said the feedback she has been getting from people in the education community is that schools are scared.
"Schools would like to know but they're terrified to know because how can you know without acting?
Gordon said she has been contacted by senior students at two other schools in New Zealand, one is public and the other private.
"I was contacted by the head girl at one and deputy head girl at another. They said we think we need this survey at our school, can you help us?
"I can't talk much about this at this stage but I do believe surveys are going ahead."
Gordon said she has not been contacted by any boys' schools.
"I'm disappointed that schools haven't engaged in the debate. They've closed their shutters.
The first-ever national youth health and well-being survey, WhatAboutMe?, is being delivered to 12-18-year-olds across the country now.
Questions cover a range of well-being topics as well as sensitive ones such as self-harm, pornography, consensual sexual activity, unwanted sexual contact and where young people go for help with sex and relationships.
The survey is currently being completed by students and data is expected to be available in the first half of next year.