Get online support
I'm posting this in case anyone in a similar situation to me needs to know how to get this combination of things to work.
* NBN Fibre to the Node (TFFN) connection
* TPG NBN Account
* Telstra Business Smart Modem
It's an odd combo for sure, but I got the modem off a friend who hadn't used it and I thought I'd save some money and some eWaste and see if I could get it going with a TPG NBN FTTN connection. Behind the Telstra branding this modem is a pretty decent Netgear (V7610) router with a fast-ish processor.
After some fiddling around with settings on the modem admin portal I got it to work. Here's my settings.
Advanced >> Setup >> xDSL Settings
Advanced >> Setup >> xDSL Settings
Ignore the IP address values (leave them blank) they will get filled in when you connect.
The cabling was pretty straight forward, but if you're buying 2nd hand you'll need the seller to have kept the black splitter box you can see in this photo.
Then plug the black DSL cable out of the splitter into the wall and the grey one into the DSL port on the far left at the back of the modem.