Stand far enough away to be safe but close enough for the violent person to see you and be aware that they are being watched and heard
Take note of where you are, what time it is, how many people are involved and what they look like. Take film/photo of perpetrator BUTONLY IF SAFE TO DO SO
Intervene verbally by calling out, 'Hey, what are you doing?', 'That's not on!' and/or 'I'm calling the police!'
Get the support of others: Ask other people who are nearby to help by calling 000 for instance.
Ask the victim if s/he needs help. Ask if they are okay? Do they need a taxi or uber? An ambulance? The police?
Report crime information anonymously to Crime Stoppers online or by calling 1800 333 000.