Madeline Anello-Kitzmiller was shocked at the negative reaction after getting glitter art painted on her chest on New Year's Eve at Rhythm & Vines.
The 20-year-old American visitor says she was yelled at by fellow revellers at the Gisborne festival before being grabbed in an incident caught on camera.
The footage has gone viral since it was uploaded on Facebook today.
The footage, posted by a woman unknown to Anello-Kitzmiller, shows her walking through the throng topless, with friend Kiri-Ann Hatfield as the man comes up behind her.
He grabs her before running off and sitting down with his friends on the grass.
Anello-Kitzmiller then spins around, races after the man and slaps him in the head four times.
Hatfield then pours a drink on him.
Anello-Kitzmiller, spoke to the Herald tonight while travelling home from Gisborne to Muriwai, west Auckland, where she lives with her Kiwi boyfriend Max Ashworth, 21.
She said she had had her chest painted along with Ashworth's sister, Katie, 18, at a gypsy stall at the festival.
"Within five minutes, we were being told we were disgusting, people were yelling 'put a shirt on'. I couldn't believe it. It was just as much girls as it was guys."
The day before, she'd worn a sheer top which showed her nipples and also had negative reaction.
"So I had a lot of built up anger.
"I was walking to my campsite and saw this hand come up. He got a handful of my boob. I went over and hit him.
"It was quite shocking. I'm used to love and kindness, freedom of expression, equality."
She went back to her tent with her boyfriend to talk about what had happened.
"I was going to cover up but I didn't want to let them win. My body and Katie's body is beautiful. It's completely natural. We were born naked."
Anello-Kitzmiller said she carried on partying until 6am today.
She described the experience as a "culture shock" - she is originally from Portland in the US where she says people are more liberal at festivals.
"I remember we went to a festival where there was a giant mud pit and people ended up naked, caked in mud and there was no negativity."
By 7pm today, the video of the Gisborne incident had been viewed 238,000 times and shared 1200 times, after only being live for five hours.
It attracted several comments with people weighing in.
"It's been crazy," Anello-Kitzmiller said.
"I didn't think it would explode so much. I had no idea the video had even been taken."
Her boyfriend, who lived with her in Portland for six months last year, said people at festivals there "literally walk around completely naked without anyone noticing or caring".
He responded to the naysayers on Facebook saying he was "the proudest boyfriend ever" and shocked at the reaction.
"What the f*** happened to treating people with love and respect? I'm so disappointed that people are still so ignorant here.
"Madeline you are a bad ass and I'm so proud of you for sticking up for yourself, you are an inspiration to so many people, keep doing what you do.
"P.S she didn't let that ruin our night we literally danced til sunrise and had the best night ever despite everything."
The footage was posted by Giann Reece who said: "Idk who you are glitter booby girl, but you and your sassy friend were the highlight of my RNV experience. Hope you and your party breasts made it home safe".
She added the hashtag: #lookbutdonttouch
Rhythm and Vines spokesman Chris Henry said he wasn't aware of the incident.
"I have spoken to the event manager and this isn't something they are aware of. The health and well-being of festival goers is the biggest priority to the team and a complaint like this would be taken very seriously."
A police spokeswoman said she was also not aware of the incident.