Subjective question, but generally speaking, it shouldn't be too much of an issue for those who have received a 99+ ATAR, provided they are willing to put in the work.
University is a different style of learning and assessment to high school, and some struggle with the adjustment, but I'd say it should be pretty manageable in a medical science degree for a high achiever.
ETA: At Griffith you should actually still be motivated to put in the work because if your GPA is in the bottom 28.5% of students in your cohort by the end of the undergraduate degree, then you'll be allocated a BMP position rather than a CSP.