The most reliable way to get two computers running on the one telstra adsl modem (presupposing you have a basic adsl modem not one of the new ones with a hub incorporated) is to first of all sign up with telstra for the extra user $11 per month (you can use the modem sharing feature in Windows but it can be a bit unreliable at times - $11 is cheap and is a much more reliable connection). Then disconnect from telstra and disconnect the adsl modem network cable from the computer and use that cable to connect the adsl modem up to the "uplink" port on your lan hub (usually the far right port - make sure the uplink button is pressed in).
Check that your first computer can still connect to the Internet and then install the adsl software that you were supplied with by Telstra on the the second computer. Setup a new profile for the second user using the new login details and away you go!!
