NAPLAN is a rubbish measure that is standardised and does not consider personal circumstances at the time of test taking, disadvantage etc.
I am also fairly sure it does not include students who were absent on the day of a particular test (e.g. numeracy, spelling, writing etc). It can also be manipulated, in the sense that some schools dedicate weeks to prepare for the NAPLAN entirely (I know a few that do this, just for them to look good on paper). That would indicate an ability to rote-learn rather than genuine achievement.
I don't believe NAPLAN results accurately indicate or predict success at VCE. There is a massive gap between Year 9 (the final year the NAPLAN is conducted) vs. end of Year 12 when ATARs/VCE results are released. Obviously, somebody who achieves high in NAPLAN (e.g. a Band 10) is more likely to do well come VCE, but it certainly is not guaranteed.