Everything is done to save money. This will save them shitloads. Not having to prepare, administer and mark mid year exams will be a lot more financially viable. And AFAIK, the GAT is (or at least was) outsourced to ACER, so this leaves them having to only run the end of year exams.
From an educational perspective, I think it sucks. Having to memorise a years worth of work will be tough.
That having been said, I am all for long (3 hour) exams, especially in the sciences. 3 hour exams are pretty much standard in first year uni science subjects. Moreover, whilst the may seem harder at the surface, I would argue that they are easier. 3 hours is a long time so you can manage your time better. In a short exam, if you have a bad 10 or 15 mins, it is very difficult to recover from it. 3 hours on the other hand gives you more flexibility for the occasional stuff up, which takes the pressure off a little.