Bystanders to bullying perpetration are considered an extremely important group to engage in bullying prevention and intervention. It is important to understand the key differences between students who are proactive bystanders, who try to stop the bullying and those bystanders who observe but take no action to help the student being bullied. Of 1,231 secondary students (aged 11 to 15 years) surveyed in 12 Australian schools, only 26.9% (509) reported they had not witnessed any bullying. The actions taken by student witnesses were grouped into proactive bystanding (786, 41.5%) and passive bystanding (445, 23.5%) responses. Age, gender, victimisation, being connected to school, perceived peer support and moral engagement were examined. The strongest predictor for proactive bystander responses was previous experience of bullying victimisation. Feeling connected to school, having higher levels of peer support and being morally engaged also predicted proactive bystander behaviour. Age and gender were not associated with being a proactive bystander or a passive bystander. Implications for school policy and practices to prevent and reduce peer bullying behaviour are discussed.