Sounds like some basic troubleshooting would be a good place to start. It will help rule out a range of possible causes such as a line fault. So I would suggest starting with http://telstra.com/fix
If it identifies a fault, then it will provide you with a reference number which will then mean you won't have to go back over the same troubleshooting with tech support.
Also checking to see if there is an outage here: http://servicestatus.telstra.com
Also what method are you using to test the download speed?