It ultimately comes down to personal preferences, but I would personally use it for a one-way F/R from PEK to SYD or v.v. and fly Suites. It's 78625 miles per one-way.

Or, if you're willing to find separate flights for HKG-PEK, you can also get round-trip SYD-HKG r/t on SQ in J for less than 100k miles.

To me (and everyone is different of course) 90k miles for AUD 700 value is crazy. If you redeemed 90k for Suites LHR-SIN one-way, it will be more like AUD 7000 value (granted, you may not be willing to pay AUD 7000 anyway, but the point still remains - you can get a lot more value for the same 90k miles).

For me, spending miles are all about redeeming tickets that I would never pay cash for, which means F or J only. For Y, the difference is usually so marginal that I might as well pay the cash fare and earn the miles.