A spate of teenage fights have broken out, a month after a school brawl at a suburban train station.
Up to 60 students were involved in the fight at Orakei train station on 24 July.
Two of them were arrested and charged with assault, and several others were suspended from school for several days.
Inspector Gary Davey said this week's fights have been happening in public places at peak times and in front of commuters, after being organised on social media.
"There's a serious danger that someone's going to get seriously injured or even killed through these fights and, as I said, it's not always the punch that will kill somebody, it will be when their head hits the ground."
Mr Davey said the police will come down hard on any students who are seen causing or taking part in violence.