2 unit has changed, but not in a long time. I think the last change was early to mid 80s when they added Plane Geometry. At some point before that, Trig Expansions such as sin(A+B) was moved to 3 unit. Other than that, I can't think of any major changes since the HSC began in 1976.
In Extension 1, 3D Coordinate Geometry was removed (not 3D geometry) and was replaced by Circle Geometry. This was when 2 unit picked up plane geometry. They also dropped the sum to product and product to sum results at about the same time. I think that is about all.
In Extension 2, the entire syllabus was rewritten in 1981, and it has been more or less the same syllabus since then. The syllabus document was rewritten in 1991 to provide a lot more detail, but the extra detail pretty much described what was already being asked in HSC exams, and so cannot be thought of as a new syllabus. So past papers back to 1981 are very useful. Before that they are not particularly useful.
