TV3 does a 3 day forecast at the end of the weather and uses 3 letter codes for each city. For some reason they chose to use WTN for Wellington where I think it's more appropriate to use WLG. After all that's the airline code for Wellington. They use AKL for Auckland not AKN.
I realise not all cities in the forecast have airports but if they do, why not use the code that most people are used to?