expired Telstra 1GB Mobile Broadband for $19/Mth, 3GB for $29/Mth if You Have an ABN
On the 21st Jan, Telstra will refresh their mobile Next G data rates for their "Telstra Business Mobile Broadband". This offer is available for anyone with an ABN.
On a 24month contract, if you bring your own modem:
1GB for $19/month
3GB for $29/month
You can get a free modem if you pay:
1GB for $29/month
3GB for $39/month
I believe you can use either 3G or Next G modems, though obviously the connection speed may differ.
You might find cheaper service elsewhere, but Telstra service is probably the most reliable, fastest, and with the widest coverage.