Was the issue with checked baggage or carry on? (or both?)
Given it is SQ ticketed and (presumably) the long haul sectors were on SQ metal (?) the SQ is the Most Significant Carrier and under IATA Rule 302 and so
SQ's checked baggage rules apply for the entire journey.
https://www.airnewzealand.co.nz/most...ficant-carrier
Less clear to me is the situation with carry on, and the 'allowance' might depend on other factors. The AirNZ website notes:
This applies to flights operated by Air New Zealand. If part of your journey includes codeshare flights or other airlines, please check their allowance as well.
Did the pax have NZ status or were they Koru?
Was their AirNZ ff# entered into the booking? or another?
Was the NZ domestic sector under SQ code share? or native NZ code?