Do you have more than one PC in the house? have you tried another PC connected directly to the router? if there is no line fault, and 3 modems are doing the same thing it could possibly be a PC problem.
Other things to try
1. in command prompt type
ipconfig /release hit enter then
ipconfig /renew and hit enter again
.
2. While still in command prompt type
ping www.google.comthen ping your router 192.168.1.1 or 192.168.0.1 or if a telstra router 10.0.0.138
3. Check your ADSL Filter by swapping the phone and adsl lead, if your adsl drops out immediatly your filter is fine. (dont forget to swap back)
4. disable firewall and antivirus - then goto
www.ozspeedtest.comand see what speeds your getting, screen shot it if the speeds are below par and email them to telstra.. also when you ring 13POND and you get the recorded message just say "Operator" it will then put you through to a service rep without having to jump through the normal hoops
hope something works