There is an exisiting outage that might be causing issues with the authentication:
National - ADSL Login - Current Interruption
Some ADSL customers may be unable to access the internet or register.
Start: 01 Mar 2016 11:23
ETA End: 03 Mar 2016 19:00
There may be a more specific issue for your specific area however that would require your location so you can check outages and resolutions for yourself here; http://servicestatus.telstra.com/
National - ADSL Login - Current Interruption
Some ADSL customers may be unable to access the internet or register.
Start: 01 Mar 2016 11:23
ETA End: 03 Mar 2016 19:00
There may be a more specific issue for your specific area however that would require your location so you can check outages and resolutions for yourself here; http://servicestatus.telstra.com/