You should expect 2 word counts, one for the HSC (40 minutes each essay) and internal exams (50 minutes).
For 40 minutes, 800-900 words is extremely comfortable as you're going to have to write 3 of them in 2 hours.
For 50 minutes, 900-1100 words could do but you'd have to still practice.
I'd say if you want a solid answer grab an essay you've done in the past and try and write it word for word with a stopwatch and see how many words you can on average write in those time periods.