If part of the plan is that Eden Park is demolished, any new Waterfront Stadium will have to have a similar capacity in the 50,000-60,000 range. And you think it is most likely EP would be demolished sitting on highly valuable land, in a city with an ongoing housing shortage.
You need a National Stadium minimum 50K capacity to give NZ any chance of staging future big events like co hosting a mens Football World Cup (1-2 pools), Rugby World Cup etc etc. You'd fill it twice a year with say 2 ABs test matches, and then numerous times over the summer with music concerts. Unless Helen Clark has a Viaduct apartment the potential to host concerts, the Crusty Demons or whoever should be unlimited.
Again the model I bang on about is BC Place where the lower tier is basically on wheels and is moved close to the sidelines for Vancouver Whitecaps MLS games, so to create a more intimate true football feel. Looking at photos they hang huge sheets down from the roof covering the upper tiers, trying to create atmosphere/sound. BC Place hosts like over 200 different events per year, to help make it a successful business model. Vancouver is about the same size as Auckland as a city.
Any new stadium will have to be far more than just a sports stadia to make it viable.
Edit - I doubt the economics would stack up, and there probably isn't enough land space in Auckland's waterfront anyway. But another model is Cardiff, with Cardiff Arms Park (15,000) right next door to the National Millenium Stadium (74,000).