No no no. I am sure you are mistaken Xiao. I just dropped INFS1000 and I am not considered 'part-time'. A number of my friends are also doing 18crpts and are also considered 'full-time'.
From the Commerce Handbook:
Undergraduate students
Undergraduate students are normally considered as full-time if
they have a HECS weighting of at least 0.375 each semester.
Anything under this amount is considered a part-time study load.
Note that some faculties have minimum study load requirements
for satisfactory progress.
The Arts Handbook simply describes it as anything less than 18 credit points is part-time.
Simple way to check would be to look at your online enrolment in MyUni. Navigate your way to the page where you can vary your enrolment online...and it will indicate what your current status is - part-time or full-time. But I am sure you are fine.