I'm not 100% certain on the passing rate to be honest. But I do believe they send around 50-60 candidates that have passed stage 2 down to Adelaide for flight grading. Each CP course caps at 18 cadets. You can do the maths! But as I said, I'm not sure exactly how many they send down for FG as it is conducted in batches. (ie CPXXBatchX)
You should note that the selection process of the Cadet Pilot Program is non competitive, which means that given you meet the CX standards, you'll be offered a place in the program.
unless they have an ultra high standard that destroys 2/3 of the candidates... i don't see how 60 ppl and 18 spots can be non competitive. ...