I got in for year 9 entry,where all I had to do was the exam and place in the top 5% in the school.
In my cohort, they took in around 20 students for year 11 entry.
Year 11 entry may be a bit more competitive or may be less depending to which school you to. For me, more because my old school wasn't that strong.
Just make sure your general numeracy and literacy skills are strong. Learn more vocabulary, because it will help you with the verbal reasoning aspect.
For the essay writing section, practice under the time constraints. Markers don't expect masterpieces in 15 minutes, the most important thing is that your language sounds free flowing and natural.
Lastly if you get through the exam, you have interviews. Although I didn't have to do interviews to get in during year 9, I have done interview for scholarships and accelerated classes. A few of my friends did interviews for principal's discretion and they were with the assistant principal. Just have a good reason for why you really want to go to Macrob (eg. academically challenging environment, like minded people etc.), and mention special achievements (sport, music, leadership, anything else unique).