Lacho,
This thread contains a lot of good information on what to expect at both specialist testing and Assessment day so make sure to give it a thorough read through. As for the essays, I am under the impression they are utilised by the ADF to assess your ability to communicate effectively. Adding to what Jimmy recard said, it would certainly be a good idea to think about your personal motivation, expectations and qualities for the officer role prior to heading in as time will certainly be of the essence. However, equally as important in written communication is structure (you don't want to be jumping all over the place) and especially, spelling and grammar. Effective communication is a pivotal part of the officer role so expect your language to be heavily scrutinised. If you're looking for some feedback I'd suggest writing some mini-essays and getting your English teacher to have a look at them.
Good luck!
Kyll