This might be quite subjective but how hard is it really to get a high distinction?
Would you be interested in an Arts perspective?
Yes! 
To start off with something disappointing, I think you're right - it is quite subjective. It's a bit like me saying, "Hey Reus, how hard is it to get into a Bachelor of Science/Bachelor of Global Studies double degree at Monash?" You might think, "Well, that depends on a whole lot of stuff." That's how this is, too. But I'll try to answer your question anyway, because it's something that I was also curious about in my first semester.
In my opinion, an HD is the equivalent of an A+ in Year 12: not easy to obtain in the greater scheme of things, but by no means impossible if you put some work into it. The thing with uni (most units, at least) is that your final grade is made up of multiple assessments. In that regard, it's a bit like a study score. So the question is, really, "How hard is it to get an HD [80%] on any given piece of assessment?" On the assumption that you will perform reasonably similarly across assessments, this is probably an easier question to answer.
In Arts, getting an HD may not be as difficult as you first imagine, but there are some things that you have to consider. Firstly, it's important to
follow the criteria of any given class, assignment, exam or reading. In terms of grades, it doesn't really matter what you
want to write or what you actually believe in; what matters is how you respond to the question, and whether or not you satisfy the criterion/criteria.
That is probably broader advice than just assignments, too. Let's say that the criteria of getting an HD are working relatively hard, going to your classes, revising a bit for exams and doing the readings. That sounds pretty straight forward. That's because it
is pretty straight forward. Honestly, there's not much more that you need to do. It's kind of like Year 12 - much easier to do well in with hindsight. Have you ever thought "Wow, if I just did x and y, I could have completely aced VCE"? It's like that.
The most subjective bit that I have listed, I suppose, is the 'working relatively hard' bit. For me, that means attending all of my classes (and taking decent notes), doing most of the readings (and taking decent notes), making sure that I hit every single criterion for my assignments (taking decent notes is not needed, here), and revising during SWOTVAC for exams (this is where those decent notes come in handy). It will obviously change from faculty to faculty; I know that Arts has a very minimal workload compared to Science and Law and Medicine (yeah, yeah - to everything). But that doesn't mean that you can just expect to rake in the HDs; you still need to fulfil what is expected of you.
I guess the take-home point here is that it's not too difficult
on the assumption that you metaphorically tick all of those boxes. Do the readings, go to class, make sure your assignments are of a good standard, revise for exams.
All the best.
