I don't think it is a very wise idea at all. Sydney is a city 4 times the size of Auckland, and it gets by very well on a single airport. I think that if traffic split between AKL and a new airport, it would most likely result in both running at a loss. Running one airport for international and one for domestic could be another alternative, but what is the point? Manukau has plenty of room for expansion. And it would be very inconvenient to have to traipse right across the city to get from an international to a domestic flight.
What is your source on this Mike? Who is proposing this new airport, and where do they intend on placing it?
There was some talk last year of using Whenuapai as a secondary airport, targeted at low cost carriers. I guess this could work, but I have a feeling Virgin would rather operate out of the main airport to secure transiting passengers.
I do think that growth of Auckland International is inevitable - the sooner they get on and build this second runway the better. I was a bit disappointed that it won't be able to handle anything bigger than a 737/A32x type aircraft, but then again, I suppose they don't really need two big runways, at the moment anyway. I think it is only a matter of time before the terminal development continues, possibly building a second pier adjacent to the current one. That was what the strategic plan originally proposed (anyone else remember that Vision 2010 thing? I remember at the time thinking how long ago that seemed - we must be two thirds of the way there now in terms of time! I imagine it could well be renamed Vision 2020 given the current rate of the way they are doing things.)
As for the South Island, the recent attitude of airlines has been rather disappointing, but I predict that over the next few years flights will be ramped up. Christchurch is a great airport, with plenty of room to expand. It reminds me a lot of Melbourne in its general configuration. Hopefully they will be able to price themselves competitively and gain some more international traffic.
It would be nice to see more traffic at Queenstown as well, but unfortunately the airport isn't in a particularly great location, and the runway can't be expanded any further. It can't even take 767s without serious restrictions due to the lack of run off areas at the end of the runway. I personally think the great expanse of flat land next to the Remarkables, on the other side of the Kawarau river from the airport would make a good site for a long runway, and a satellite terminal. Control facilities and customs etc could be kept at the Frankton site, with an airside rail connection to this location. I don't know how feasible my idea is though - given the terrain and everything, and also how much traffic would use it.
Another place to keep an eye on is Wellington. The airport there is in quite possible the worst place possible: it is unable to expand any further, and there are serious doubts over its safety with the roadways and water at either end of the runway, as well as the rocks at the Lyall Bay end that would pretty much break apart an overrunning aircraft. There has been talk for a number of years about building an airport in Paraparaumu or somewhere close (beyond the hills that surround Wellington city itself). Such an airport would be able to handle larger aircraft such as 777s or 747s, which Wellington can't under normal circumstances. It would be linked by rail to the city centre. I don't know what the latest is on this proposal - the last I heard was that an American businessman was promoting the idea, but this was well over a year ago.
Finally, Hamilton has some potential, perhaps as a secondary airport for Auckland, but as with the case of a second airport in Auckland itself, I don't think there is enough of a market there is to split between 2 major airports. However, I wouldn't be too surprised to see some transtasman flights from Qantas or Air NZ to Hamilton sometime in the next 10 years.
"So powerful is the light of unity that it can illuminate the whole earth." - Bahá'u'lláh