There is a lot of misinformation on the net generally and on a lot of RC forums especially re the maximum height RC aircraft can be flown at.
After some recent discussions with Bad Angel & Hoppy I decided to chase down the ACTUAL CASA regulations pertaining to this.
Whilst you need to read ALL the relevant rules to understand what approved area means (I read them all and it takes quite a while to chase down all the meanings etc) the section that refers to the height is as follows.
CASA Regulations Part 101, Subpart 101G – 101.400
101.400 Operation of model aircraft outside approved areas
(1) A person may operate a model aircraft outside an approved area above 400ft AGL only if he or she:
(a) keeps it in sight; and
(b) keeps it clear of populous areas.
The rules are apparently currently under review in line with new technologies (eg; FPV etc)