I think about 60 people turned up to the semi structured interview on the 5th of Jan ( like 15 were music/med, some interstates and a few others were born in Australia but did exams overseas). According to the site there are 30 spots for domestic applicants and 10 for international (although those born in Australia are apparently still domestic since they are born here regardless if they did an international exam).
FROM USYD WEBSITE:
Places available
Up to 30 domestic places will be offered each year for the combined medicine degrees.
There are up to 10 places available for international applicants who meet the requirements.
Up to 5 additional places will be offered each year to applicants who identify as Aboriginal or Torres Strait Islander people in the following combined degrees:
Bachelor of Medical Science / Doctor of Medicine
Bachelor of Science (Advanced) / Doctor of Medicine
In terms of preparation I don't know theres really anything you can do apart from not being overly reserved and emotionless during the interview. It is very, very casual and is more like just a nice talk than an interview.