Meant to be closed book.
This happened at my school with very rampant cheating (i.e. over half the grade cheated including a proportion who had the intention then chickened out, ridiculously high averages). Pretty sure the same scenario applied to the yr 12's at my school.
I was pretty pissed, my ranks were way below what they should have been, albeit this was for yr 11 and not yr 12.
As a parent, I would be pretty mad and would complain to the school, which I assume is what Thomidog is considering. However, do note that the school is inclined to not overturn those exams unless sufficient proof is provided of cheating occurring. Make sure that if the cheating is revealed, that your kid is anonymous and not labelled as the 'snitch'.
Personally, I doubt complaining would be very fruitful, unless it is a collective action with a decent amount of evidence. It is also likely too late to complain now. Given that your child is able, ranks could have definitely been regained in the following assessment tasks. If however, the marks are the difference between being a doctor/lawyer, it might be advisable to complain and bring it to P&C etc.