Seeing how most students don't really prepare at all for Year 9 NAPLAN, as opposed to HSC where nearly all Year 12 students are likely to prepare well (because the stakes are much much higher), it's definitely not a reliable indicator in my view. One is more likely to do HSC past papers in Year 12 than NAPLAN past papers in Year 9.
Furthermore, NAPLAN only tests English (including writing) and Mathematics while HSC students are often taking on 4-6 subjects. There's a good amount of HSC students in the past who have attained strong ATARs without taking mathematics in Year 12 (yet it's compulsory in NAPLAN).
NAPLAN English is also different to HSC English. NAPLAN English has MC questions while HSC English doesn't. NAPLAN writing sections are often done on the spot while in Year 12, most students prepare material in advance for their HSC English essays.
There might be a correlation between NAPLAN results and Year 12 results, according to research, but I don't believe that it's a reliable predictor.
I remember getting like 83 for School Certificate (when it existed) English in Year 10 yet 91 in HSC English in Year 12.
SC English was very much similar to NAPLAN English (that being said, I remember getting mainly Band 10s for Year 9 NAPLAN English sections - except for reading which I got a band 9 lol).
