Hangacat, welcome to Airservices!!! I thought they were the written tests you are doing this time around... what computer failures could you be talking about?
If this is abinitio recruitment, aviation quals aren't important (but very handy when training).
The best tip: Speed & Accuracy. Don't be put off if you are only 1/2 way through a test when the time is up... so long as you are accurate and correct that is important. I vaguely remember that marks are deducted for incorrect answers in most tests.
Dress code: casual. ATC is casual dress, except in Brisbane Centre
Seriously though it's only in the interview (if you get that far) that appearance may be a factor.
Most imporantly, a relaxed - not complacent - attitude as you go in. Ignore the others.