All those cross-border trucks have drivers, just like the ships importing stuff into New Zealand have crew. Unlike ships, the trucks don't just stop at the border and unload - they go on to destinations within the country, use trucking facilities along the route, etc. It's the people that're the main risk rather than the goods they carry. (Though when trying to eliminate the virus altogether even rare events can be a problem. There's some evidence that Covid may have spread to a NZ quarantine hotel worker via a contaminated surface and nearly escaped into the community. Obviously, the bigger the country the more likely rare events are to occur too.)