For all complaints to Auspost - my local post office does not accept or process complaints, they refer one to the national number/web site. The staff are otherwise very good and they claim it is Auspost policy.
1. get the name of the person who took the complaint if it is not in writing. Make notes for future use.
2. get a transaction number
3. specifically state you want feedback about the resolution/followup to the complaint and ask who you should followup with if you do not receive it (by a date they suggest). If you don't, they probably won't.
In my experience their complaint handling varies from good to lip service, and if you do not try to hold them accountable they might not be accountable. My post office claims that contract drivers with 3 valid complaints against them are terminated. Based on posts all over the internet I question this as most able bodied drivers might have already been employed at Auspost for a short time.