I know the VCAA have that policy in place that no marks are changed after they are given, but if there is a gross injustice, ie. the marks on your exam added up to 79/90 and they accidently gave you 69/90 (i think it would be doubled so it would be 138 instead of 158) then they would have to change it, no doubt about it. They can't say, we made a massive mistake with adding up your marks but we aren't gonna fix it. It would lead to HUGE lawsuits with people claiming that VTAC hindered there chance to get into University.
But the fact is that the above scenario is VERY rare. Unless you are VERY VERY VERY sure that they mucked up then dont bother. If you think that they should have given you more marks for working out or something, you almost certainly won't get allocated more marks.