Hi saralee, I usually bring the quantity that my baby is going to drink + 1 or 2 extra just in case (depending of the length of the flight). There is no policy on the quantity you can bring on the plane. You can bring one tin of formula milk if you want. But dont't bring the water. You ask for water once in the plane.
I know that you can't bring food into Australia. You may check with the Australian ambassy what's the policy for baby formula.