All circuit traffic calls should be done on the CTAF only.
The Centre frequency you are referring to is the Air Traffic Services (usually Melbourne Centre ML CEN) available at the location. Due to limited range, it can be either in the "circuit area(yscr)", "on the ground(ypjt)" or a specified altitude e.g. 3000ft(yspk).
This is given FYI in case you need it for example to cancel SAR and you dont have a phone on you.
(its mainly used for IFR traffic that need to contact ATS as early as possible with their taxi/departure calls and SAR on arrival)
Have fun and be safe! (oh -and it might be a good idea to have a good long chat to the boys at the flight school to give you a more thorough brief, a fair few things have changed recently such as scrapping GAAP aerodromes, circuit procedures in CTAFs etc)