Ambah,
I really understand your question and your concern.
My son turned six this year and did prep (he goes to an independent school). He could have gone to grade one but I really am a believer that kids need to play a bit more. He is a sensitive little soul but academically advanced for his age - I wanted him to just play and have fun (prep is play based learning). I made the right choice for him and he has loved every second of prep. I have seen many little boys especially begin school earlier than when they were ready for formal education and that has been to their detriment.

So, being in Qld I do know there is some scope for choice - well at least that was my experience... I hope that helps.