Hi guys,
I went through the same process for the Sydney open day...(sitting infront of the computer at the moment with a very sore arm from all my needle jabs ouch!) and had the same question about "what would you change about Australia".
I think they are looking for someone that puts their ideas across but also listens to others. We had a guy in our group that was bringing out all these statitics about the government and really pushing his views across and he didn't make it to the next round. Also don't tell someone that their idea is wrong. Maybe say like that was great but how about this etc etc.
They don't care whether you get the answer right or wrong as long as you can show why you came to that answer and that you worked as a group to get through it.
Also keep and eye on the time as they only give you 10 minutes to choose three topics and you don't want to spend all the time on the 1st idea.
If you don't make it maybe its not your time. They told us that the reason people were cut was because they were not ready to be cabin crew.
Good luck