I'm at Macquarie, and it depends on the unit.However normally when we get to 'pick' we roughly 3 minutes to find a group so it doesn't feel like picking really.
Sometimes it's randomly allocated or you choose. I did psychology and some computing courses, they choose it for you. I did some other courses like arts and other computing courses (COMP2121), you get to choose.
It depends on the subject, but in general, if the subject is a first year subject, often they pick groups for you in case you still don't know your cohort very well. The later years you tend to get to pick your own groups as you get to know more people.