I think you'll find that being older for your year is better than being one of the youngest especially when to comes to high school . If she is only average in some subjects you may find that she is then behind if she were to move up a grade. If she was ahead in everything yes sure, but you say she is not so I'd be tempted to leave it. And seriously, it's Year 2 not high school. If she has already made friends and settled after a move from another country you risk unsettling her further. I speak from experience having changed countries myself when I was 12.
Children are often 8 or turning 8 in Year 2. You'll see as a previous poster has said that there can be as much as 18 months difference between ages in a class so she is with her peer group. Though she would also be in peer group in Year 3 if at the younger end of the spectrum.
It's also not common and quite rare to move children mid year in Australia even in private schools. Private schools often have a lower class size ration than public schools and there may not be room for her in Year 3 - which may have been another consideration when the school placed her. If she is very advanced they are more likely to skip her next year straight to Year 4.
While you may feel she is not being challenged intellectually, she may have a few social problems skipping up a grade mid-year. Children are often not particular kind to a child jumping grades.
If you fear she is not being challenged there are programmes like Kumon that run externally from the school she could participate in.They tend to focus on Maths and English. And this would definitely challenge her.
We have a son who is one of the older kids in his year, he could've been the youngest in an higher year but we decided we'd rather he be 18 in Year 12 rather than being 17. We felt we'd rather he was one of the kids with a drivers license, rather than being driven by mates. Strange reason, but there are a lot of accidents with young male drivers carting their friends around.( And we live regionally where the problem is worse)
As an aside unless the school runs from K-12, I'd be hauling ass to get a placement in a private High School. Many of these are booked out from very early ages, and you may struggle finding her a place unless you get moving now.
For @eatCheese. NSW
"Children can begin compulsory Kindergarten at the beginning of the school year if they turn five on or before 31 July in that year. By law, all children must be enrolled in school by the time they turn six."
Again, being very young for your year, whether or not he has have already completed a certain amount of schooling may not be to his advantage long term.
See more at www.schools.nsw.edu.au - this site might give you a bit more clarity about it.