~~It's a shame the clip doesn't begin a second or two earlier, it would help paint a clearer picture.~~
^ Edit: GreenLeft posted another longer clip of the same video which shows the officer running over at 1:10
Reporting OP reply with more context:
The NDIS rally was a bit of a mess: there was a Falun Gong event going on at the same time and location, they'd already set up well before anyone got there, rally organisers did not want to move, Falun Gong had a marching band arriving as the rally was supposed to start, there was a great deal of argy bargy just getting the parade to wait for disabled people to move over to the north side of the SLV.
And their band played through many of the speakers.
Anyway, 10-15 minutes after the rally concluded David Limbrick the Libertarian Party MLC showed up to address the Falun Gong rally. Some of the people lingering around recognised him and started heckling him, and then police started arresting.
And you see what happened next: as they're escorting someone away out of nowhere one of the cops runs out of their escort cordon to knock an old man to the ground, someone tries to stop him further assaulting the old bloke, and he gets pulled to the ground and punched and maced.
The old guy was then completely forgotten by police and walked away, raising the question what was this all about?
One witness says they saw cops knock people over into the path of an oncoming tram: https://www.instagram.com/p/DYG4B4dTUeS/
Interview with the cyclist: https://instagram.com/p/DYHLnXZz0x-/