The school commissioned the survey after concerns were raised around the rise in harassment towards students.
She said everyone has a right to feel safe and clearly they don't.
Additional support has been put in place at the school for both staff and students on Monday as O'Neill said she expects the survey results will be triggering for some.
The school is encouraging parents and caregivers to talk to their teen, to start a conversation.
The survey was sent to 1042 consented students and 725 participated, a response rate of 71.2 per cent.
430 of the participants noted they had been harassed - 59.3 per cent of those who took part.
About 42 per cent stated it had occurred two to five times but a quarter had been harassed more than 10 times.
Harassment included verbal, space, written and physical or sexual contact.
The survey found men constitute 91 per cent of the identified sexual harassers, including young men the same age as the students and older men.
Most common events were cat-calling, body shaming and being rated on looks.
To May this year, 381 respondents reported 2677 incidents of sexual harassment - seven per student who had experienced harassment.
Most incidents were carried out by lone males, with one quarter by groups.
Students were asked to describe their "worst" incident of sexual harassment. Over 20 students described being raped by individuals or groups.
It appears that none of the rapes reported in the survey were reported to police.
Many other incidents involved young males at social events, on the streets or on public transport - egged on by friends, respondents said many comments were extreme and terrifying.
Participants described many incidents of physical and sexual abuse. Almost the same number of events were caused by older males on the streets, either alone or in groups, often in cars.
Older males also harassed students on public transport and in taxis and Ubers.
The worst incidents stirred up many feelings. Students were uncomfortable, nervous, degraded, upset, embarrassed and afraid, among other feelings.
Only a tiny number, less than 10 per cent, received any help or support. Most did not mention asking for help.
Head girl, Amiria Tikao, said she is not surprised by the survey results as she and her peers live with sexual harassment every day – and have for years.
"The behaviours that were identified have been pretty normalised, especially throughout my past years at high school. So when I read the survey, it was nothing I hadn't known before."
She said the results are still very disappointing.
"For all girls and the LGBTQ community, this issue is absolutely much bigger than secondary school.
"It was good to get a number because of the conversation that was started at our school. It's about carrying on that conversation and getting actual digits to put to the student's concerns and issues."
Changing aspects of their lives
More than 60 per cent of those who had experienced sexual harassment have changed aspects of their lives to try to ensure it does not happen again, the survey found.
"The most common change is clothing, with students donning baggy clothing, jackets, shorts under skirts, trousers and other additional clothing to try to deflect attention."
Many have changed their routes home or become hyper-alert about who is on the streets.
A significant number reported being harassed on school and town buses and work hard to change their routes, times, and bus habits.
Many have stopped using buses at all.
"Another strategy is to avoid boys, especially in groups. They might change direction, keep their heads down, pretend to talk on the phone and not go to particular places.
"Other changes include no longer drinking alcohol, not going to parties, learning self-defence, never being alone and keeping their phone handy."
A list of recommendations were included in the survey results, such as the school looking to a whole-of-school strategy of safe disclosure of all sexual harassment.
"Students should be encouraged to report all incidents of sexual harassment and to do so safely."
Another recommendation was that CGHS students use the cameras and videos on their phones to record all incidents of abuse.
Where to get help:
• If it's an emergency and you feel that you or someone else is at risk, call 111.
• If you've ever experienced sexual assault or abuse and need to talk to someone call the confidential crisis helpline Safe to Talk on: 0800 044 334 or text 4334.
• Alternatively contact your local police station.
• If you have been abused, remember it's not your fault.
