The relevant law is the Surveillence Devices Act. For video you should be fine, however as pointed out, audio can be tricky. The basics for NSW are that you can not record any private conversation without the consent of at least one principle party to the conversation. In your circumstances however, the definition of "private conversation" is very important:
private conversation means any words spoken by one person to another person or to other persons in circumstances that may reasonably be taken to indicate that any of those persons desires the words to be listened to only:
(a) by themselves, or
(b) by themselves and by some other person who has the consent, express or implied, of all of those persons to do so,
but does not include a conversation made in any circumstances in which the parties to it ought reasonably to expect that it might be overheard by someone else.
There are two things to note:
1. consent may be express or implied, and
2. if it is reasonable to expect that the conversation would be overheard, then is not a private conversation for the purpose of the Act.
In an auditorium or similar type of environment, it would be difficult to argue that a normal conversation wouldn't be overheard - but there's never any guarantees when it comes to the law.
As long as you don't put your microphones in a position where they are likely to pick up conversations that are clearly intended to be private, you should be fine. If the microphones are in the open and they happen to pick up "ambient" conversation, then there is a reasonabe expectation that the conversations would be overheard.
If you are recording presentations and the microphones are out in the room, then using directional microphones and positioning them as high as possible will help cut down on the ambient noise.
So as
@Tim W pointed out, a clearly visible sign on entry should suffice and keep you covered. I'll leave it to the experts to suggest what wording you should use.