I did it on several occasions throughout my bachelor degree.
* A 3rd year unit in my 2nd year
* Two postgrad units in my final (3rd) year
Units are done in a specific order for a reason (as defined in a course map) - given that most units require pre-requisite knowledge in order to be successful in them. In saying that though, if you feel that you would be able to adequately do well in it.. then I don't see why not. Forget the whole '1st year' '2nd year' etc.... just because you're in a "lower" year than others, it doesn't mean you are less or anything like that (it's not like high school
).
I actually enjoyed the challenge of a higher unit - and did quite well in them. Everyone's different, but sometimes the difficulty is a motivator and positive rather than the opposite. Just remember that you can basically 'try before you buy' for the first 3 or so weeks of the semester (can withdraw/change before census date).