Thanks for the tip Gdub!
I didn't realise that the 2 litre only came out in RS and Mexico vehicles in the UK... That clarifies everything then. Still, I contacted King Springs Australia and quizzed them on front spring ID and OD for their 2 litre springs - 110mm ID and 134mm OD.
Rang Burtons in the UK and quizzed them too, the sales rep on the phone had no idea, BUT went and measured them up for me straight away

Came back with 109mm ID and 133mm OD. Close enough!
So for anyone out there considering ordering springs from Burtons (cos they're cheap and come in various spring rates and heights) - non-RS/Mexico springs are the ones you want for AUS 1600 struts and RS/Mexico struts suit our 2 litre struts. Sorted :)