Page 37 on covers unmanned free balloons.
I think we would be covered as an light balloon - no more than 2m at any point in it's flight (would have to check the physics for how it would be at it's apex) and less than 4kg of payload.
We must inform CASA at least 24hrs before the scheduled time, as stated here:
A person must not release a light balloon outside an
approved area unless the person gives CASA the information required by
Table 101.5 at least 24 hours before the proposed release.
Penalty: ^insert units^ penalty units.
Note 1 A person can comply with this requirement by telling:
(a) if the person is an approved aviation administration
organisation — the Australian NOTAM Office; or
(b) the appropriate approved aviation administration organisation.
The name, address and telephone number of the person who will
release the balloon (or, if several people will be involved, the
name, address and telephone number of the person who will
coordinate the release)
The date and time the release is to begin
Where it is to be carried out
The estimated size and mass of the balloon’s payload
If more than 1 balloon is to be released at a time, how many
balloons are to be released at the time