I don't think you need to book if you aren't sleeping in a bunkhouse, though even if you are camping it would be a good idea to give them a call or drop them an email from the Astrofest website. For day trips its just turn up, but you will still have to register for insurance, safety etc purposes. The odd person (not meaning they are odd
) sleep in their car.
(in house Iceinspace Qld Astrofest joke. If they read this "they" will know who I mean
)