At USyd (in particular for combined degrees) you're allowed to overload in study units for intermediate and senior years (but this does not change the maximum number of units you're allowed to do).
Assuming you get good marks in first year, you can ask for permission to do one extra subject each semester than normal. Assuming you get good marks and permission granted each time, this allows you to get through your degree in a slightly shorter timespan at the price of doing an extra unit per semester.
Also, for a select few units of study, they offer Summer School, and you can complete (i.e. cram) a unit of study during the summer holidays.
This should at most save you one semester in your degree...lol
In particular for maths students, if you're good enough to do Talented Students Program in a Science degree, you can do second and third year maths subjects in first year etc...