Half an hour earlier, about 10pm, Miss Cooper had joined Makayla Spiers (18) at Tae Flavell's Cargill St flat for a Saturday night drink.
The women say they went outside and noticed a large group of teenage girls coming towards them, knocking over wheelie bins and scattering rubbish.
After the flat's wheelie bin was knocked over, the women said they asked the group to clean up the mess.
The dozen girls - estimated to be aged 15 to 16 - then formed a semicircle around them.
''What are you going to do about it?'' one of the girls asked as the group started shoulder barging the young women.
A girl then opened an RTD can and poured it over the three women as another girl punched Miss Cooper in the face.
''I was then hit in my glasses, and I saw Annalise getting pulled by the hair and being hit and kneed in the face,'' a visibly upset Miss Flavell (19) recalled.
''I ran over and was then grabbed by the hair and punched and forced over, which was when I broke my ankle.''
Both women were on the ground as they were surrounded by the drunk-looking girls, who then started to kick and punch them.
''I was screaming and this girl ran up and punched me in the face ... As I curled up on the ground she kicked me in the ribs,'' Miss Flavell said.
Meanwhile, Miss Spiers, who recognised the girls from her former school, was not targeted by the group but tried to stop them raining blows on the pair.
''It was just continuous kicking and punching to the body, head ribs, just everywhere. I would pull one girl off and another would start ... I was just useless.''
The video - believed to be filmed by a young male accompanying the group - had since surfaced on Facebook, which left the trio feeling ''disgusted''.
''Who would watch something that severe and not do anything,'' Miss Spiers asked.
The group left swearing and mocking their victims as they headed off in the direction of the CBD. The three women were later taken to Dunedin Hospital by ambulance.
In addition to bruising, the women were missing clumps of hair and had been off work, with Miss Flavell off for up to six weeks with a broken ankle.
The assault had left them shaken, suffering nightmares and questioning parents who would allow their underage teens to drink alcohol unsupervised on city streets.
Police said they were had been working to interview the six or seven alleged assailants, all believed to be aged 15 and 16 years.
Police were also investigating information that the group of assailants were involved in an earlier incident at a party.
