Hmmm. My recent credit for NZ001 LHR-AKL was about 11,400 miles, whereas
Milemarker shows 11,900 for LHR-LAX + LAX-AKL, which is how the flight is operated. So that would appear to be a direct calculation because of the single flight number. Not sure if this is firm policy though.
One possible way around this would be to credit one of the sectors to your account, leave the next one blank and then send in the boarding card retrospectively to see if an agent simply credits them separately.