If you're a domestic student you can study for a PhD part-time. You'll need to provide the reasons why you need to study part-time when applying for admission, because the University wants to ensure that you'll have enough time to complete your PhD.
It's also possible to change to part-time study during your PhD, but again a reason would need to be given and formal approval sought by completing a Request for Change of Research Details form, accessible from the Graduate Research School Forms, Policies and Guidelines page. You can only change to part-time from the start of a semester (1 January or 1 July for PhD students).
If you're an international student you need to check with the Student Visa Services team at the International office (email email@example.com) to determine if you're able to study part-time. Typically a student visa does not permit part-time study.