Does anyone know of the process for registering from interstate - in this instance SA?

The car in question has current SA registration - has a 1600 case - lowered, 135/205 tyre combination - there is nothing else that stands out (at this stage) that shout out at this being difficult to register.

But what do I do once I have the car in NSW? Blue slip?

yep , proof of ownership etc

yep blueslip & SA rego papers & a very clear receipt to you with owners full details & Chassis number & Engine number listed on the receipt

When I bought my unregistered Dual Cab in Vic, it was quite interesting the process I needed to go through to drive it back to Qld. For $30, over the phone, with no proof to suggest this thing was roadworthy or even existed, Vic roads gave me a 14 day permit to drive from Vic to Qld. Then once in Qld, I was able to register it with proof of ownership and safety slip.

In NSW and Qld I would have needed to front the Transport department with the car and safety slip to be granted a 14 permit to drive to my home address, whether that be interstate or round the block.

In my circumstances, it was perfect in terms of hassle free, but there really needs to be a National standard for this sort of thing. But hey, it took them a century to get the railway gauge the same!