Line speed has very little to do with your provider, its a function of the quality of the phone lines between your house and the exchange and your proximity to the exchange. In other words it's almost entirely under Telstras control, and changing ISP will probably make very little difference. In the last 5 6 years I've had the following approximate speeds at various places I've lived:
12MBit (Exetel, St Leonards exchange)
4MBit (Internode, Petersham exchange)
16 MBit (TPG, Richmond exchange)
13Mbit (iiNet, Redfern exchange, about 1km)
in every case there was a pretty obvious correlation between line speed and download performance and both the distance to the exchange and the genral state of the lines/wiring on the property. Ie if you live in a a street of old houses several (4+) km from the exchange, your speed will probably be rubbish.
You can use this link to check your proximity to your exchange:
http://www.adsl2exchanges.com.au/