If AIP says to do one thing and you tell people to do something else, then yes they would be in contravention of AIP.
For fly-ins where a large number of aircraft are expected and the likelihood of increased chatter on an FIA is likely and thereby present a problem to other airspace users, the procedure is to do what I note the organisers apparently attempted to do i.e.
- contact their local CASA office and request a NOTAM be published and in particular a tempo CTAF allocation (there are specific frequencies reserved for airshows/fly-ins).
Now, in the case of this particular fly-in, why whoever in CASA refused the latter aspect is probably subject to an internal "please explain"
It makes me wonder if the local CASA (VIC/TAS) office was involved, or instead CASA"s Office of Airspace Regulation.