Do you mean withdrawing from your entire degree, or just one course from your degree? I can't imagine it would effect your entrance. I was under the impression that they only took your university GPA for entrance after one full year of study, but as you have not done that theoretically they will just go of your UAI. Although, I am not sure if this is a national trend or just a NSW/ACT thing. I was also under the impression that your transcripts would have the courses with "withdrawal without failure" noted by them. It may be worth contacting the university you plan to attend next year and inquiring as to their admissions procedure. Do it ASAP though - and call not email.
I noticed that you are at ANU, and just reminding you that last day for course withdrawal is tomorrow. You cannot do it through ISIS, you must fill out and lodge a form.