This is for my school so it could be different - it's worth looking at the syllabus btw for content.
Prelim Modern is pretty easy ngl. The first term is just historiography and historical skills etc. Because of that, your first assessment is an independent research project which you come up with, research and write. Then in Term 2 you start doing a historical event- we did the Cuban Revolution and Cuban Missile Crisis. The assessment for that was a source analysis test which they gave us the sources for and just had unseen questions on the day. Then you obviously have your Prelim Exam- for modern it was actually super easy as long as you knew the content.
Year 12 is much more content-heavy. Every term you're trying to finish a full quarter of your syllabus so it gets really hard to stay up-to-date with study notes etc if you're not consistent. The assessments are still pretty easy, we had a source analysis for the Core, then for the National Study we did a presentation and then an in-class essay for Conflict. Then you obviously have the trials which are pretty tough but you really need to have a good study schedule happening and you'll be fine.
Modern is so good but it's also quite hard in Year 12 so if you do it make sure you stay on top of everything, get drafts handed in etc. and you'll go really well. GL!!