Sylent,
Have you tried Electronics Boutique they have 16 stores in Victoria, they may have some old copies kicking around.
I purchased a copy from the TheGamesWizards in NSW for about $29- last year but I don't think they have any stores in Vic.
However Military Simulations is based in Victoria
http://www.milsims.com.au/
134 Cochranes Road, Moorabbin Vic 3189 Australia
PO Box 1164, Moorabin Vic 3189 Australia
Phone Australia 03 9555 8886 Fax 03 9553 3339
Here it is in their online catalogue:
AD&D PLANESCAPE: Torment. SSI. Set in the city of Sigil, but you can also visit the other planes.You play a scarred, amnesiac immortal in search of his identity. You must discover the secret of your death and rebirth. Your own actions define your character's character, not a character generation screen. Explore Sigil, a vast city of magic and industry. Manage an adventuring party of the strangest allies. This is not a game which you have to play for 20 hours before you can cast spells. You are a power to be reckoned with (as is most everybody else in Sigil!) IBM Requires: Pen 200 MMX, Windows '95, 32mbRAM, 8spCD-ROM, 4mbSVGA. IBM - $33.00
If you do get the game vist
http://www.sorcerers.net/Games/Torment/index.htm
for some helpful files and info.