You might still have peak HLZ-WLG and HLZ-CHC flights. But outside of that they will diminish as land transport improves. Like if you built a proper high speed rail connection to connect Auckland and Wellington in a few hours you'd effectively destroy WLG as an international airport. In the true long term I expect NZ to cut back to two major commercial airports, one for each island. With smaller airports/airstrips for the occasional point to point service or local/regional use. But that timeframe is decades+ and probably beyond our current lifespans.
I doubt it - even with a high speed rail (where there is absolutely zero chance in the next 100 years) getting traffic from Wellington explain to me why anyone would use the train to AKL to fly to SYD/MEL/BNE when non-stop services exist? The better surface connection argument (such as HLZ-AKL or PMR-WLG) only works when the regional port (HLZ or PMR) cant sustain their own services at a reasonable and comparable cost to the other airport (AKL or WLG). This is definitely not the case for WLG. How cheap do you expect international flights from AKL to become compared with WLG to encourage this inefficient behaviour?
And as for your argument of two main airports in New Zealand - again I just dont get it. So Wellingtonians will fly WLG-AKL-NPE or WLG-CHC-NSN? Are you suggesting that because the roads are so good a person that is currently flying TRG-TIU via WLG will instead drive TRG-AKL then fly AKL-CHC to then drive CHC-TIU? Sorry maybe I'm not understanding what you are suggesting.
I probably wasn't clear enough then.
I'm not thinking so much that we end up with only two airports. More that we will end up with two airports that do international traffic. In the medium term I expect battery electric will become standard for the sort of short haul stuff that is done domestically. And at some point I also expect kerosene to start being taxed to account for the climate damage that's currently ignored which will likely encourage concentration of hyrdrocarbon powered aircraft at hubs. Now some of this is probably wishful thinking, but I do think at least some of it will happen.
More likely we'll continue to see commercial passenger aircraft operations be focused at fewer larger regional airports as land transport improves. Though the wildcard could be battery electric aircraft for doing regional hops into larger centers.
As for true high speed rail. I can only think of one single path that would ever make sense. Wellington to Auckland via Palmy, Taupo/Rotorua, and Hamilton. So yeah, I agree the chance of it happening is probably sweet bugger all.
Much more likely and much more needed is higher speed regional rail. With some major investment we can get rail going on cape gauge up to 160kph which would probably destroy some of the regional routes.