Great project for a great cause!
I can't really help much but I read up on this a few years ago and I believe you don't need permission from CASA if the payload is under 200g. My memory is not great though so it could have been 200g for the payload including the balloon and rigging etc. so you should definitely check. Also even if you don't need permission it would definitely be a good idea to inform CASA and perhaps get recommendations for safe launch sites.
Good luck
