Kristo, based on experience it is the bank spot sell rate, today's being 1.1239. You can get cash from an ATM on arrival and/or use credit cards in most (but wise to ask first) taxi cabs. If you want to have cash with you take no more than $60 as the fare is normally about $50-55 from the airport to the city.
A couple of trips ago I had a funny experience with their currency. As you know their $1 and $2 coins are the opposite of ours, with the higher denomination being (sensibly) larger. I got to the hotel and the fare was $54, so I gave the driver all of my Kiwi currency, $50 in notes and two small coins. Thinking I had given him a small tip I said that he could keep the change...........It dawned on me as he was driving off that I had given him the correct money and so I understood why he said "uh, thenks a lot, you Ozzies are reel ginerous eh?"