If you are really keen on Mitcham Girls I would let your middle daughter go in to year 6 with kids her age and then apply for the school as part of the normal process in year 7. Personally I would be wary of putting a child in a year above their peers unless they are intellectually, socially and emotionally advanced. And I say that as the parent of a child assessed as gifted kept with his 'proper' year group.
Some high schools run programs for kids that are intellectually advanced that extends them educationally without putting them in with much older kids. There is the ignite program that takes kids from year 8 in three schools across Adelaide, but also schools run 'in house' programs as well. I know Norwood Morialta has a program that starts in year 10 (referred to as 10X) and I'm sure I have heard talk of similar programs in other schools, although they are not widely publicised.