I'm pretty biased because I absolutely love Australian History. I did the 3/4 subject in Yr 11 and this year did an Australian History/Politics enhancement subject.
Basically if you are interested in it, it will be an easier and more enjoyable subject to study. It really isn't a very challenging course (compared to Revolutions!). Depending on what you study and your background, it is pretty relatable and very easy to find wider resources - and, in my opinion, knowing more about the country you live in is always a plus. Now everything I see in the news or issues facing Australia is filtered through my knowledge.
It shouldn't be too hard to get over 40. AustHist was my first history subject, I managed an average 45 for SAC marks, but bombed on the exam because I was told to study the wrong thing, so got just under a 40. Ugh.
The fact that you are a good English student will help in history, for sure.
Downsides of AustHist are that many students find it a boring, mundane subject. Personally I think it all depends on your teacher, and your own interest in the subject...so, it's all up to you!
Hope I helped =)