Also anyone who believes a child is in need of protection can contact the department of communities, child safety and disability services with their concerns. If they think the child needs protection they can make an application to the children's court for a protection order. This will normally say where the child should live - which may be with a relative or foster parent.
Once a child is 16 they will normally not be forced to return home by authorities as long as they have a safe place to go and can support themselves financially. Centrelink benefits are usually available once you turn 16.
If you are willing I would give your niece the option of going to live with you or another relative and if need be contact the Dept of Communities.