Thanks for responding. The original fares weren't in any promotion, as far as I am aware. Both booked within a month, but the flights were 4mo apart.

Tried taking this up with 'customer services', but even after berating their generic initial response I just got:
>>
Thank you for your email.

Unfortunately, we are unable to comment or respond to queries regarding business decisions behind our fare structures and pricing as these are confidential and privileged to Air New Zealand only.

If you are experiencing issues with our site please feel free to contact Online Support either via email or on 0800 35 22 66.

We appreciate your understanding.
<<

i.e. another generic repsonse!

This is a shame as it now looks like my relationship with NZ has to end, just as it was looking so promising!!

Nevermind, there seem to be newly fitted planes coming on stream from other airlines (UA, AC to name just the *A) and hopefully the 'open skies' agreement will open up some better routes with new airlines and help keep prices in check