1. This is the full tracking of your parcel:
China Post Register Air Mail Tracking System
Parcel No.RJ767688736CN tracking result
2016-06-18 19:49:21.0 Hangzhou city international Transit Station received
2016-06-18 19:49:49.0 Hangzhou city international Transit Station customs scan
2016-06-18 21:50:07.0 leaving Hangzhou city international Transit Station ，next station Hangzhou center
2016-06-19 07:42:15.0 Hangzhou Transit Station export security scan
2016-06-19 16:34:44.0 Hangzhou Transit Station export customs cleared
2016-06-22 21:04:00.0 arrive Beijing Airport
2016-06-22 21:32:00.0 leaving Beijing Airport going to next airport/seaport

2. Technically there is no problem with the tracking and parcel should have arrive in Australia.

3. Have you phone Australian Post or just look on their site. Because if you just look on their site they are not tracking this type of parcel (register airmail) from China.

4. If you still have buyer's protection, just open a dispute because you never receive the parcel. If seller told you that he did investigate, normally he should have told you that the parcel was sign and deliver. Even me I can tell you that (your parcel arrive in Australia it's been over 2 months).

5. Also your parcel could have been seize by Australian Customs, but I suspect they would have sent you a letter telling you so. My feeling is the parcel is lost.

Hope this help.