No, Dont land a cable in Whangarei, Not even a branch - It's quite stupid. Northland has ~50k of homes or around 150k of people. It's tiny and building a branch here is costly along with offering no diversity since we have no power generation up here and need Auckland for power/comms/everything.
As for landing anywhere else if you cant make the case for a 2nd cable coming into NZ at all why is there a case for landing it in the south island or lower north island? There isn't.
National transit costs's aren't too bad either - Certainly not MORE than INT unless you're talking about delivery to bluff. I can pick up INT @ $35/mbit in AKL and transit to Whangarei at $4/mbit or NAT Transit WhangareiAucklandWelligton @$13 - And thats at 1gbit buys, You wanna haul 10gbit around and it gets cheaper again.
Also Don, Cables normally dont have 50-100 year lifespans, More like 20-30 so that $75mill spend now? Gonna have to go it again before you even kick the bucket and you dont even factor in O&M cost's like having a crew ready to repair it should some farmer dig it up at 5am. O&M over the lifespan of a cable can be upwards of 25% of the cable build cost