I've got pretty much the same set up as you describe. I had all sorts of problems getting the ADSL sharing sorted out (particularly as my "server" is running Windows 2000 Server which is "not supported" by Telstra). In the end with a bit of help from various sources I got it working and the office now has 6 puters sharing the one ADSl connection (without having to pay the extra $11 per month per puter).
I found the following link to be very useful. Hope it helps:
