At AKL this is only the case for customers connecting onto a New Zealand domestic flight. (There is one exception - flight NZ303 to CHC, which is effectively an international flight, customs and immigration processing is done at CHC).

International-International at AKL does not require the full customs and agriculture treatment, but as cavemanzk points out there is a security check (like at SYD) to go up to departures and the transit desk from the arrival concourse. Showing your ticket would be sufficient to get past here I suspect.

There are plenty of reasons why you might not have an onward boarding pass (flight not yet open, suspended check-in, inexperienced staffer) but they still have effective measures to ensure only the right bags travel. I'm sure we've all been in delay situations where they remove bags when a passenger hasn't turned up for instance.