What's the fastest - whilst still being trackable - way to send documents to NZIS in Wellington?

I need to send our passports from the UK. I have a holiday booked so I can't send the passports off straight away. I've left 6 weeks between sending the passports off for blue stickers and having to be on the flight (Jan 2nd). Hopefully this is plenty of time, but if I can get the passports there the fastest way, I'm sure it would help.

All advice welcome.

lnz