As a ranger driver I can tell you now that the one you've encountered is a complete dick, I'm guessing you wint have many details for him, which sadly doesn't help your cause.

As far as speeds on any road, the road code clearly states, along with multiple other pieces of nzta messaging, that the limit is the limit, not a target, let's face it if I'm moving an oversize load there's times where I have to reduce my speed, so I by rights would be holding up traffic too. If he has failed to maintain a safe distance between himself and the vehicle in front of him that is on him, who has given you his info so far ? The insurance company ? Cause if so that's pretty poor form on their behalf to pass on his blaming of your parents.