surfisup1000:
One question I have, is why it costs so much to tunnel in NZ.
I think it's a matter of geology.
Iceland is volcanic, which basically means solid rock. NZ (and certainly the Manawatu Gorge) is mostly sedimentary, with lots of weak unconsolidated rock. So almost everywhere you tunnel in NZ you have to do a lot more to support the roof and deal with water seepage.