Talk to the local council that covers the area in which the event will be held, and talk to the liquor licensing people at the state government. They'll tell you what licences and permissions you need, what restrictions you need to observe and so on. If it looks like you'll get past the local and state requirements, talk to the ATO. They should be fine if there was no money involved and the quantities small, but may have issues as there's an entry fee to the event and the quantity suggests something more than say a club trying to get new members. Be as open as possible, provide full disclosure to all bodes you talk to, and ensure you understand and strictly adhere to any conditions. If you can't get them to provide you with written responses, keep notes, get the name of the person you spoke to and the call reference number if there is one.
It'll take time and effort, and you may end up not being able to do it, but make sure you get it right. The fines and penalties for getting it wrong are draconian.