Of the major vendors of pre-paid mobile solutions, Vodafone and Optus offer the most price-competitive packages.  If you had to make a choice between the two, a good rule of thumb is to see what company most of your friends are on - and follow that company.  This is because in this way you would be taking advantage of the free / low rates of calling between 2 phones on the same network.  However, Optus has a "Fair Go" policy that warns that if you "overuse" their "Free Time" service, they could suspend your account - so take heed if you do hear a warning.  Further, both Optus and Vodafone offer plans together with mobile phones.

Telstra is expensive on all accounts.  It also does not offer a wide range of plan options.  However, the selling point is greatest convenience, especially for recharging.  Unlike vodafone and Optus, Telstra allows you to recharge over the Internet and phone as well as in person. 

Optus offers the Ericsson T10 as part of their pre-paid mobile package