Marston Moor were not particularly helpful in finding out what happened. Consumer Guarantees Act states it is the responsibility to ensure goods are received by the customer. Contract law states that when a supplier engages a sub-contractor, in this case NZ Post, they are solely responsible for the actions of that sub-contractor.
Marston Moor left entirely up to me to follow through. That it took a very specific request to get information from Marston Moor so I could file a written complaint with NZ Post is disappointing. Further they suggested phoning instead (no surprises they would suggest the laziest option) when writing carries far more weight.
I will not use again, and have now ordered my own import of bags.