Mine is similar; LGW-DXB-AKL is LGW-DXB-BNE-AKL. I currently have the option to upgrade from DXB-AKL, but not the specific DXB-BNE or BNE-AKL legs, they are shown as one leg, probably because the flight number is the same? Guess you'll have to do this over the phone rather than online, but I'd be interested to hear what happens.