Yup what both mystikal and jimmy said. There are different rules for each faculty, heck, even different rules for each unit, it depends on the unit coordinator themselves.
Eg, I got the position to tutor AFC2340 next sem because I started talking to Andrew Sanford (who takes the unit) during the semester break before the year even started about the unit. Then continued communication with him by email throughout the semester and got to know him very well. In the process, I told him about my experiences at teaching and how I was really passionate for the unit, he also checked my scores (WAM/GPA) and said there will be slots available next sem (as he is firing some tutors who aren't doing a very good job lol!) so told me I will be first to get the available slots.
Some other units are difficult to get the job, like AFC3540, the unit coordinator never checks email, it's hard to grab hold of him after lectures to even chat, so yeah it all really depends on the unit/faculty.
Other factors come into play as well, eg, I'm very good friends with my AFC2340 tutor this sem (I've had him for at least one subject ever since first year), he's been on the job for 3 years now and is one of the best tutors for the subject (and other finance subjects too), all of the lecturers really trust him, so I had him refer me as well which helped alot.