I assume the next round of interview in Australia will start October-November.
Studying CPL’s will show a level of commitment to becoming a pilot which is exactly what they are looking for. As well as looking for someone who fits the profile of a cadet they want to make sure that applying for this programme isn't just a passing phase, which could lead to someone leaving the programme half way through because they were bored and had not thought out their career path properly.
As for preparing for the knowledge test (I assume you mean the JKI) it essentially should help, however in the interview (stage 1) you will be asked questions on all the theory you should have completed so far in your studies that you are qualified to. i.e if you have passed CPL Met you will be asked some meteorology questions to a standard of CPL.