I dont go to UNSW, but usually there is a final date in each semester when u can actually add or withdraw from subjects. So if that date is past, which is most probable then u have to wait till next semester starts. If UNSW does internal transfers then u can most probably fill forms and stuff to transfer to eng/arts next sem or either u will be told to come back after this sessions results then u can transfer.
Most importantly is that u should organise consultation with either faculty to discuss ur matter.