Will this be a repeat of the last thread on "theory instruction requirements"?
In truth, I would be surprised if you could pass all exams with less than 15hours of study per exam. There are however little proof required by ATOs to ensure the requirements have been upheld. The hour breakdown is also a recommendation and although preferable to have structured tuition, I don't see that enshrined in the requirements. To enforce my point of view (which may well be wrong) there are no fields on the SRG1105A that require proof of the theoretical instruction requirements having been met.
I received no more than around 60 hours of structured ground school including flight briefing time, but spent well over 100hours self study, and practice. The ATO let me sit the exams when I felt ready for them, not when I had ticked a certain number of hours box.