Just email him and ask if he is aware that it is illegal to post anything containing a lithium battery overseas so the fact he unlawfully sent it to NZ negates any claim he may have against you and suggest he contacts Aus Post or whichever courier he used to see if they caused any damage while dealing with a non compliant package.

By the way you should not have sent it express post either, anything with a lithium battery must be sent regular parcel with a road only sticker on it.