First of all, there is a bureau in every state and in addition to that, there is also an outbound bureau.
Any QSL cards sent to you via the bureau are received by the WIA and forwarded on to the local state contacts on a regular basis. In turn, each state bureau distributes the cards to the local amateurs, ether directly if you're a high-volume user and Alek lives near you, or indirectly, via local clubs, ham-fests and other gatherings.
Once a year any cards that haven't been distributed are sent to you directly, but only if you're a member of the WIA.
On the outbound leg, that is, cards sent by you to other stations, the QSL bureau works a little differently. You send your cards to the outbound bureau in New South Wales. You should have a look at the WIA page on how that specifically works.
You're expected to sort and group your outbound cards and if you're a member of an affiliated club, you can group all the cards from all the members together and send them in batches to the outbound bureau. I'd have a look at the weight limitations from Australia Post and make sure the batch just the right size for the weight category, making sure you minimise the postage costs.
If you'd like to get in touch with Alek, you can, firstname.lastname@example.org is the address.