Hi Frappe,
Try calling the Defence Switchboard on 1300 333 362, they should be able to connect you to your closest base.
To add to what VS recommended regarding practice aptitude tests I found these helpful in preparing:
Assesment, IQ and Aptitude Test Training
How to pass aptitude tests with example tests
Defence International Training Centre
Aptitude test questions
Apologies if some of these links have been posted before.....