the naplan results are the most viable pieces of evidence you have cause they are really the only external exams youve recently sat.
*My percentile ranking against the rest of the state in my year 9 naplan closely (relatively anyway) mirrors my ss from last year
I disagree with the idea that NAPLAN could (or should) even loosely be used to judge your standing within the state in Year 12. NAPLAN/Year 9 level content is probably distant enough from Year 12 to the extent that you can't really reliably infer much at all from it.
NAPLAN's purpose and the way that test is designed is also quite different to how exams for VCE subjects are designed. Even for the GAT, which I guess is somewhat similar to the NAPLAN, it isn't all that often that there's a clear relationship with study scores achieved and GAT scores.
If anything, the most viable piece of evidence you could use to figure out where you stand is probably if you feel like you understand the content you've covered so far well enough that you'd be able to do well on the exam (though even with that you end up learning a lot in that last month of exam revision).