Thanks for your advice it is much appreciated. 

AP could only verbally confirm the weight was correct and when requested to provide evidence in their system of this again could only verbally do this with 'there were no discrepancies detected'. They were however able to provide proof of delivery and signature (although I suspect this is the drivers doing given COVID). So the buyer has now opened an INR claim. 

Can anyone advise on best way of responding to this? I have uploaded tracking again (although this was already done) and outlined AP confirming weight correct and 'proof of delivery/signature'.  I want to also get eBay to look into his account for previous issues. As I have noted to the buyer we have over 1200 transactions on this account and only once have we had a claim like this from a Policeman in WA (although he acknowledged clear evidence of tampering). Australia Post will not pay out the $200 insurance as 'they fulfilled their contract'. I would really appreciate some input.