I is the bucket you are after for requesting classic awards as a Platinum.
You have a reasonable chance of it being approved, July isn't usually a busy month on the SYD-HND route as it's unbearably hot & humid in most of Japan, and not a holiday period for them so little AU-bound tourist traffic.
Worth having a read of the wiki article on WP/P1 award releases:
Preferential Access to Qantas Classic Reward Seats (wiki content) - Australian Frequent Flyer
If you'd consider departing on 24/7, both QF25 and QF79 (MEL-HND) have I9. If you'd consider departing on 26/7, QF25 is I9 also.