Why is Wellington so windy?

Wellington is known in New Zealand, and around the world, as 'Windy Wellington' due to the frequent strong, gusty northerlies that Wellington experiences. A hardened Wellingtonian is used to the strong winds, and may even miss them when they do disappear for a few days. So, why does Wellington get such sustained strong winds? There are a number of factors which come into play that make Wellington 'the windiest city' in the world with 178 days a year gusting at or above 63 km/h.