The cheapest option if you have a Telstra Next G phone and don't use a lot of data is to take the sim card out of the phone and put it in your modem when you want to access the internet. You can use data on your phone account on a pay as you go basis. You will lose your bigpond email address when you cancel that contract. When I did it, I had to download a turbo modem manager from
www.maxon.com.au as I couldn't get the bigpond one to work on the Telstra plan. Obviously you can't use the phone when the sim is in the modem.
I did this about a year ago. Plan fee saved $420. Data cost $0 as I don't use it much and you can use your phone call allowance to pay for the data.