Is History Extension hard? It depends what you mean by hard. This is all how it works at my school, but I think it's pretty similar in other places.
The Major Work is a long, hard road. But it's doable as long as you put your mind to it. At our school you have three terms to finish it. This means that if you slacken off in one part or take a long time in another, you can catch up pretty easily provided you put in the effort to get back on plan.
But as you get to choose the subject of your major, you'll be doing something you're really interested in, so it'll be bearable to work at for three terms or so.
The best thing about the major work is that it takes up 80% of the internal mark. This means trials and pretrials are only 10% each, so they won't be too distracting. Use them as practice for the actual HSC.
As for these exams and the actual coursework... historiography can be hard. If your teacher gives you stuff about postmodernism especially, it can really make your head go fuzzy. And there's a lot of reading. You don't have to do it all, you could do what I did and read all the stuff a week before exams... but I don't recommend it. It's hard to learn an essay for historiography and the best results you'll get are the ones you make in the exam, so it's hard to learn beforehand for it. Unlike, say, Ancient, where you can learn all the bulletpoints on the syllabus, a lot of what you'll learn in history extension is between the lines. If you can get a comprehensive understanding of all the historiographical issues, you're set.
The case study? Pretty easy compared to historiography. And because the question's so vague (it has to be applicable to every single possible case study), it's pretty easy to learn an essay beforehand, as long as you keep it flexible.
So overall, is history extension hard? If you choose a major work topic that interests you, no. And if you can understand historiography, no. But the major requires lots of work, and if you slack off during the year, you'll pay when you try to study before the HSC.