It became violent when some of the protesters broke through police lines.

It is not clear if the protesters became violent or if the police became violent or both. Crowds often have lives of their own. Seemingly slight pushes from the back can become surges at the front. The break could have been unintentional. Then again, it may well have been purposeful. I guess I'm saying that, from that article, it's not clear what happened, but basing my assumptions of Australian police on what I feel about American police, I can easily see that they were looking for an excuse to whack a few folks.