I'm guessing the waste removal program isn't optimised for raw shipping costs. They only do one ship-load per year (deliver all supplies, take all waste back).

I'm guessing the waste removal program isn't optimised for raw shipping costs. They only do one ship-load per year (deliver all supplies, take all waste back). Overall logistics (including regulatory compliance) might actually make it cheaper to have a single destination, rather than stopping over in NZ to offload some of the simpler to dispose waste. Actual shipping costs are probably in the noise floor after everything else. It's also possible the program has been partially optimised for pork barrel politics.