All our means come directly from ACARA. ACARA produce two means- the first one comes via their summary national report. This is the mean we use when we first release NAPLAN data to SPA around September each year. In around February the following year ACARA publish the final national and state means which will differ because of some further adjustments they make, SPA re-adjusts the means inside the software after the final report is produced.
Each state has been know to release their own NAPLAN state averages based on different statistical modelling. As NAPLAN is a national program administered by ACARA, we use their figures, not the state's figures. Secondly, ACARA treats outlier data more statistically correctly - a procedure which has national agreement and "smooths" the effect of having just a few too many or just a few too few students below national minimum standard.