Rhythm and Vines organisers say they are "incredibly disappointed" after a woman wearing glitter body paint was grabbed at the festival.
A video of the attack, involving Muriwai resident Madeline Anello-Kitzmiller, 20, went viral after being posted to Facebook.
Anello-Kitzmiller is seen racing after the man and slapping him in the head four times.
Rhythm and Vines event manager Dan Turner organisers "do not condone any form of harrassment".
"We are incredibly aware of the issues that arise at music events, particularly in large crowds.
"We are incredibly disappointed that an incident like this has occurred at Rhythm and Vines, and that a select few feel that it is okay to behave in this manner.
"We want our customers to enjoy themselves, in a safe environment and be able to express themselves in a way that is in keeping with other international European festivals.
Safer Venues spokesperson Kit Lawrence said he contacted festival founder Hamish Pinkham last year and offered him their services for free, including posters about what behaviour was appropriate, and advice about offering safe zones for women feeling harassed.
Lawrence said that Pinkham was initially enthusiastic, saying it would be good to "rebrand" the festival.
But Lawrence said that enthusiasm petered out, and he received a final message from Pinkham saying the posters would "negatively highlight a problem that doesn't exist".
"I've been to festivals; I know there are minor sexual assaults every year at every festival.
"But there's a difference between taking a stand, and not. You can also show the people who might feel vulnerable that somebody cares. You can say, come and talk to our staff.
"But when I explained those reasons to Hamish, he just didn't get back to me."
He said it was surprising they said no "when someone has done all the work for you".
He believed they didn't care.
But Turner said, in response to Safer Venues' offer, that "we felt a more practical approach was more appropriate".
He added: "We know the problem exists, but it's figuring out how to combat it in a practical, communicative way that allows our patrons to feel safe so we respectfully declined the offer from Kit as the festival site includes wellness areas, chill out spaces and an Angels tent for our customers, giving them the means and pathways to report any issues.
"We have also taken steps to introduce a Patron Helpline which we will roll out next year."
Lawrence said a lot of work had to be done to make music festivals enjoyable for everyone.
"Especially young guys, they sometimes don't understand that their 'japes', classic example with this at Rhythm and Vines, can have really serious consequences."