But what if I want to work less than my current role - do they have to allow that?
No they don't have to. However legislation came in a couple of years back around flexible work arrangements. Again it's case by case but they need to provide you with a good reason not to allow you more flexible arrangements (most use client commitments as a good reason not sure what kind of work you do.)
Check to see if they have a flexible work policy they probably should and if you ask and they say no- and it only applies to carer of younger than school age children you can speak to fair work Australia.