If NZ Post wanted to take this issue seriously and provide a decent service, they would issue posties with an additional mail item to drop into any letter box into which they are delivering these obvious letters. The mail item would contain a community service announcement warning about this current prevalent scam.

This provides them with a plausible deniability - NZ Post has not failed to deliver the suspicious mail item, but have chosen to mail drop a service announcement designed to prevent NZ customers being made victims of fraud. They can readily acknowledge that the mail drop is being targetted to likely victims.

That would be a simple solution that would fall within their policies.