Can you apply to a secondary school at the start of 2016 for F.5/F.4 if you've been studying in AUS for 5 years?

Certainly yes.

Suggest that you would :
- study the HK syllabus(F.3 - 5), comparing this with what you are now having in AUS,
so that you can estimate how well you can do if you are going to study in a HK sec. sch.
Also, this would help if a certain HK school invite you for an interview in future.

- consider applying to an international sch. This type of sch.might suit you more.
There are plenty in HK, some of which are quite well-run, with good public exam. results too.