The short answer
Chiropractors, osteopaths and physiotherapists all treat musculoskeletal problems — back pain, neck pain, headaches, sciatica, shoulder pain, sports injuries. All three are five- or four-year university degrees in Australia and are regulated by AHPRA.
The differences are in emphasis. Chiropractic emphasises the joint and how the nervous system responds. Osteopathy emphasises how one region of the body influences another. Physiotherapy emphasises movement, load and rehabilitation exercise. In practice, the individual clinician matters more than the letters after their name.
How to choose
Book with whichever practitioner will properly assess you, explain what they found in plain English, and give you a clear plan — treatment plus what to do between visits. If you don't get that in the first appointment, try someone else. That applies inside all three professions.