yeah do it now, i went to unsw yesterday to apply for a city rail/bus/ferry concession thing and they wouldnt give me one cos id only enrolled in sem 1.
apparently u cant be considered a full time student until ur enrolled for the full year.
Yes you should enrol into semester 2, just so the university knows you will be attending university in semester 2. You're a first year right? There isn't much choice in courses anyway, so just pick the cores and an elective. You can always change your elective later anyway.