How does a standalone honours degree work?
I am familiar with the situation in NZ. Since Australia and NZ share many commonalities in higher education, i suspect that things are similar.

A normal bachelors degree is a three-year degree and by itself not sufficient for getting into a PhD program. An honours degree adds a fourth year to the bachelor degree and is normally open to very good students (there are GPA criteria for entry). In particular, an honours degree often involves a research project and is deemed sufficient for PhD admission. Most if not all people do the honours degree immediately after their bachelor. Generally, you need a four-years degree to get into the PhD, which can either be an honours or a master degree.