On this site 95% of the people posting may have got a UAI over 95, but I'd say state-wide it's a bit closer to 5%
Also, I think the UAI includes everyone who graduated from year 10, but didn't go on to study in year 11 or 12, so in fact out of the 66,000 that undertook the HSC the percentage that got over 95 would have actually been greater than 5%