IF you are Australian citizens, you will need to access the National Disability insurance scheme....
it will be a total and massive pain, and you may get nothing - ADHD is generally not covered at all and ASD if he is high functioning and high IQ may not either...will depend.
Either way, expect to have to have bucketloads of appointments/reassessments/applications/agony.
If successful (ie his special needs are severe enough to require the assistance of an aide in school, other therapies etc) you will get a package of funding under NDIS that you can use to fund various support/therapies as needed (not entirely simple, but that's the gist).
As a basic rule of thumb (obviously lots of exceptions) you have a choice of public, private and catholic. Within public system, there are some schools with special needs units and a couple of dedicated ones, but v difficult to get a spot at the dedicated ones.
You have automatic enrolment in the state school whose catchment area you are in - request for one out of area is based on space/assessment etc.
Catholic schools are a) bastard hard to get into at the moment, b) throwing all their dosh at funding priestly sex offenders in court and c) extremely bad at managing special needs - the overall view seems to be that most disorders are basically due to a lack of discipline!
Private schools also don't have a fab track record for handling kids with special needs and tend to encourage them to move schools if proving problematic, but you will need to sound out particular schools you are interested in for a better handle on how suitable.
Public schools have some great special needs support, but limited funds, and support unit quality and availability vary widely, so again, you will need to research and if you are set on a school, live in that area.
High schools in sydney are bursting at the seams thanks to consecutive govs selling off land so it is all a bit of a mess.
Education wise - public schools can range from decent to grim. There are a handful of selective public schools too (by entrance exam) - if your boy is bright enough, he will still need at least 3 months of solid exam coaching to stand a chance, as most entrants have spent several years in intensive coaching and extension to gain a place. The difference in public (non selective) and private is pretty wide, both in terms of extras offered and educational outcomes, but as a rule of thumb, the whiter and higher socio-economic the area, the better the public school.
The above is a snapshot with a lot of generalization but gives you the gist. It's all a bit rubbish in Sydney at the moment, are you sure you want to come back?