Which science are you doing? 3970?
If you drop chem now, you would waste money since the HECS census date has passed, and if you drop later than
Friday, you will be given a fail for the subject.
Also keep in mind that you will be 6 uoc behind your program, which you will have to catch up on later. If you cannot overload to 30 units in another semester to make up for the extra 6 uoc, you will be at uni for a semester longer to finish anything left over.
You can find more comprehensive rules to the science programs in the
Science Handbook
If I were in your position (by no means is this program advice), I would be more than half way through this course, and considering I've come so far, I would push on to the end of the subject.