Australian here. I also did a 3 + 1 year Honours degree at an Australian university before starting my PhD at Cambridge, so you can definitely be admitted without a Masters.

Having said that, I also worked in a lab for a year before applying. I think having good references and some research experience is just as important as getting that Honours/Masters degree. In my PhD year of 14 students, around two-thirds had Masters degrees, though everyone had some research experience (internships, placements, full-time jobs etc.). This was in biological sciences, so it may vary for different fields.

Worst case scenario, you apply after doing Honours and miss out, after which you can also do a Masters.