The buyer seems to express himself in a slightly weird way, but this doesn't necessarily make him a scammer, and he doesn't seem to be angry either. If he has opened a case with AusPost without even telling you, he might be genuine, even if I don't understand why he accepted the parcel if it was obviously too light, but maybe the parcel arrived with other parcels or he just didn't think about it.

I have occasionally received parcels that were not perfectly sealed because the tape was not so sticky any more (maybe due to humidity sometimes?), but luckily they were not empty.