Minimums for entry are easily found, basically Year 12 pass, 18 or older and Aus or Kiwi citizen.
The diploma basically is recognition of a competency in ATC basics, both theory and practice, as per MATS
Manual of Air Traffic Services (MATS) - Airservices
Once you have done that training you go onto more training in the area you will be working in, this is called a rating.
While the basics seem well basic, reality is entry is very hard. At the time I entered there had been a hiring freeze for 3-5 years, 18000 applications were received for the 80 odd positions in the three courses to be held. During the training which was done at Henty House in Melbourne
https://planning-schemes.app.planni...ittle-Collins-Street-Melbourne-April-2022.pdf (since converted to apartments), there was a mandatory 80% pass mark on all subjects or you were out. We lost 20% of the course during this phase. We then move to our final postings for further training and the issuance of an ATC licence, we lost another 40%.