Is that 66% average a raw score or the percentile scores that MedEntry provide?
If it's a raw score, then I can't really interpret it since the difficulty of exams differs a lot.
If it's a percentile score, then technically, I'd say that 66 percentile will NOT get you into Medicine. HOWEVER, most people have not even begun to do the MedEntry exams (I hadn't until after midyears) and the ones who have actually started tend to be the more diligent, and well, smarter students. This can easily skew the percentile rankings, and just due to the low amount of students being ranked at the moment, further skew the percentiles.
Still, you've got PLENTY of time left to improve so don't worry too much yet. As for UMAT scores, the cut off last year for Monash was around 93 percentile for an interview (and I know a few who got in with this borderline UMAT so it's not a huge disadvantage), and for other unis, it's around the same (90 percentile plus?).