Why not take a look into the BA executive club? Although you say they will travel in Y i understand that BA fare ex-AMS can provide a significant discount, they could look at WT+ fares and i believe they would be able to get to AKL using solely BA codeshares (AMS-LHR-SYD-AKL) although the WT+ would of course only be applicable to the LHR-SYD. I believe they'll then earn 125% of miles flown.

They would have BA Silver/OneWorld Sapphire within 2 return flights as members residing in mainland europe only require 400 tier points to reach this level, which gives you lounge access and business class check-in when flying any OW carrier, along with a 50% tier bonus for miles.

It'd be worth checking out, and although it may bear an extra cost they'd only need 25,000 miles (i think it is) to then upgrade to Business Class (from WT+). Providing they fly WT+ on their first return flight they should have enough miles each to upgrade to Club on their next return flight.

Best bet is to check the BA board, as the guru's on there know far more than I do, and will also be able to point you in the right direction for finding cheap WT+ and Business class BA fares in europe!

Good Luck.