I think a purpose built stadium in Petone would be ideal. Wouldn't add much from my travels from Karori, and if there were dedicated trains all the better.
A bit off topic but if your going to Domodedova airport in Moscow you can check in to your flight at the train station as your boarding the train so the commute is part of your check in time. Would be ideal if the train actually goes into the stadium itself, so you check in when boarding, or can buy the tickets on the train and then depart right into the stadium, so no more gates, security, etc. There are ways to make it seamless and hassle free.
I keep thinking about Easter Road stadium in Edinburgh. Its a 20,000 seater just outside of the CBD without much in the way of public transport but they get good crowds turning up. It serves a city marginally bigger than the Wellington region but one that hosts two professional football teams (well shortly only one).