Auckland sits at roughly the same latitude as Seville or San Francisco, and the climate is mild year-round. Summer (December to February) is the best time to visit: warm and sunny with long evenings, and the harbour comes alive with sailors and ferry traffic. Expect temperatures around 23–25°C, occasionally hotter. This is also the busiest period, and the time when Rangitoto's pohutukawa trees flower spectacularly red along the coastline.
Autumn (March to May) is the locals' favourite: the summer crowds thin, the weather stays warm and often golden, and the city's cultural programme picks up. Winter (June to August) is mild by Northern Hemisphere standards — rarely below 10°C — but frequently wet and grey, and the city turns quiet. Spring (September to November) is variable but increasingly pleasant.
The America's Cup (held in Auckland when New Zealand is defending) transforms the waterfront and the city's mood entirely: check in advance if it coincides with your trip.