You DO need to accept your offer - it isn't presumed that you will.
From the UAC website:
"If your application is successful, you'll receive an offer letter and information from the institution making the offer that explains acceptance and enrolment dates and procedures. Some institutions allow you to accept your offer online.
Each institution has its own instructions on how to accept your offer. These instructions will be provided to you when you receive your offer. Follow the institution's instructions carefully.
Accept your offer by the date specified by the institution otherwise you may lose your offer. Don't let your offer lapse."
Take a look at this page
http://www.uac.edu.au/undergraduate/faq/offers.shtml for more information.
You'll get enrolment details after you accept your offer.