Video shows two groups of spectators shouting and throwing flares and road signs at each other outside Leichhardt Oval on Sunday night.
Police said the crowd started to disperse after local police and officers from the riot squad showed up at the ground about 7.30pm.
“There were noises, there were cops, it was going off. There was no security,” one bystander said.
“It was really bad, you couldn’t get out, it was a war or something like that.”
They’re investigating the incident and trying to identify those involved.
Police minister Paul Toole condemned the violence at the football game.
“People should be able to go to sport and enjoy it, he said.
“People should not feel as though they’re threatened.
“People should not feel as if they go to an event where they can enjoy a spectacle like a sporting event and actually see thuggery and this kind of behaviour.
“Rest assured if there was an incident that occurred, we will be investigating and we will get to the bottom of it.”
Anyone with information should call Crime Stoppers on 1800 333 000.