Error 651 could mean a lot of things.
You said that you are a new contracted TPG customer. I take that to mean that your previous ISP was not TPG. Now, your old ISP may have supplied you with their own modem whose default gateway is different from the one used on your new TPG modem.
For example, TPG's NBN modem HG659 has its default gateway as http://192.168.1.1
It could be that you have configured your PC to use a static IP and that static IP is pointing to your old modem default gateway.
To fix this:
In case you need to assign a static IP to your PC, just pattern it after 192.168.1.1. For example, you can use 192.168.1.4 or 192.168.1.10
For subnet mask use 255.255.255.0
Most importantly, use 192.168.1.1 as default gateway address.