Hi Chris, you have quite a few options:
1. Is there a label on the minitower near the plug input showing the polarity? Is there a AC/mains adpater to the minitower as well? There is usually a label on the AC adapter showing the output current and polarity.
2. You can plug it into an outlet and test the polarity with a multimeter. If in doubt, you can always take the cable to Dick Smith or Jay Car and they can hook it up to test it for you (if you ask very nicely).
3. If you don't have a multimeter, Jay Car acutally sell a polarity sensing 2.1mm plug
http://www.jaycar.com.au/productView...m&form=KEYWORD so you can buy this plug and use the bare wires on your existing plug to determine polarity.
HTH
Bo