States and territories have different regulations. Please, refer to the relevant authority in your state/territory. As an example, the relevant information for NSW is available in the following. link. 

https://www.facs.nsw.gov.au/families/parenting/responsibility-and-rights/children-home-alone

Please, note that people who deal with children, like teachers, have very strict rules around the safety of your children. They are required by law to call the relevant authorities if they suspect any possible danger to children. Please make sure you understand the law clearly.