This depends on your status.
If you are an international student on a student visa, you are required to maintain a full time load (100 credits) at all times to ensure your degree is completed within the standard course duration. Students who fail to maintain a full time enrolment may be reported to the Department of Immigration and Border Protection (DIBP) for breaching the conditions of their student visa. Also see what is considered as a full time load for an International Student?
Domestic students can study part time providing their course offers this mode of study. Full time enrolment for domestic students is classed as 75 credits and above. If you are a domestic student, it is at your discretion how many units you take. However, please be mindful that you have a maximum of ten years to complete your degree and that some courses have a maximum enrolment load of 75 credits. You will find course structures outlined in the Handbook.
You may find our 'What is Equivalent Full Time Student Load (EFTSL)?' FAQ helpful.