I though SA was South Australia? You mean she's Sefrican?! Bah!@ Humbug!
Kids in Sydney start school at 5.
What you can expect? I have no idea. It's a year or 2 since I left school, and I'm a kid-free zone. If you knew roughly where you were planning to live, I may (note - MAY) be able to make more sensible suggestions.
16 will probably put your kid in Year 10 (schools go up to year12).
Were you looking at public (state) or private schools (sometimes called Public, as in GPS - but only the very swanky!)
Some high schools specialise in particular area, e.g., theatrics, sport, computers, etc. Some state schools have selective examinations.
Much depends on where you end up.
Welcome to Sydney anyway! (are you a cricket fan?)