The weather in New Zealand in early summer is generally warm, even hot in some places, but is less reliable than later in the summer (January and February). Average high temperatures in the upper North Island are around 75-77° F (24-25° C), and a few degrees cooler in much of the South Island (with a few notable exceptions, such as Nelson and Alexandra).
While much of the North Island experiences lower rainfall in December than earlier in the year, and places such as Auckland and Northland are about as dry as they'll ever be, parts of the South Island are much wetter. Dunedin, Queenstown, Invercargill, and the West Coast (Hokitika, Westport) receive more rain in December than in any other month. Generally speaking it's safe to say that the further north you are, the drier the climate in December, but you'll need to pack a rain coat or umbrella wherever you go.
Crowds and Costs
December is the start of the peak tourism season throughout New Zealand. Accommodation, campsites, car rentals, and transport (including the InterIslander Ferry between Wellington and Picton) not only cost a premium, but also book out in advance.
This is especially true later in the month, when New Zealanders take their summer holidays. Schools are out for about six weeks from mid-late December, and many other workers take the week between Christmas and New Year off. It's essential to book accommodation as far advance as possible when traveling during that week. Even campsites in the most popular places can book out months in advance, as many New Zealand families have their favorites that they return to summer after summer. The roads are at their busiest at this time, too, so extra caution is required when driving.
Where to Go
Nowhere's 'bad' in New Zealand in December, so where you go should depend on your own interests. Generally speaking, for warmer, drier weather, stick to the upper North Island.
New Zealand has an abundance of very beautiful beaches, and wherever you go you won't be far from some. Northland, the Coromandel Peninsula, and the top of the South Island have a great combination of gorgeous white-sand beaches and sunny weather, so these are good places to head if you want to swim or enjoy water sports. Northland's Bay of Islands is perennially popular (for good reason), so if you want to avoid the crowds, go a little further north.
Travelers often overlook the bottom of the South Island, and Rakiura/Stewart Island off the south coast. But December is a great time to travel to this beautiful area. While temperatures are cooler than further north, they're warmer than they will be at any other time of year. This is a great region to head to avoid the crowds that you'll find elsewhere in December. The Catlins area of Otago/Southland is a highlight.
What to Do
Many travelers come to New Zealand in the summer to enjoy the beautiful beaches. If you're after white or golden sands and ideal swimming conditions, stick to the east coast of either island, as beaches down the length of the west coast tend to be more wild and less suitable for swimming (but are still very beautiful)
If you're an experienced hiker, December is a good time to try some of the higher-altitude treks. You'll need to book accommodation in Department of Conservation (DOC) huts on the most popular trails ahead of time, as this is peak season. But temperatures tend to be cooler at higher altitudes, so walking for long distances in December shouldn't be too uncomfortable. Be aware that extreme weather, such as snow, can occur in the mountains in December, so be prepared.
Christmas and New Year are major holidays in New Zealand, and many New Zealanders have the week between them off work. Christmas tends to be a family time, and a lot of attractions shut at least for the day. You'll find big parties, in both small towns and bigger cities, for New Year's Eve on 31st December, though. The small city of Gisborne, on the east coast of the North Island, is the first place in the world to see the new year, so has especially fun parties.
Traveling to New Zealand in December? Check out these great itineraries
Highlights of New Zealand - 10 Days. This action-packed itinerary dives into New Zealand's North and South Islands, offering a wide variety of landscapes and attractions.
New Zealand's North Island - 6 Days. From Auckland to Taupo, this six-day itinerary presents the highlights of the North Island's diverse and dramatic landscapes. Trek to emerald lakes and past ancient volcanoes, explore The Shire from Lord of the Rings, and discover the sacred thermal waters of Wai-O-Tapu.
New Zealand North Island Self-Drive and Cycling Tour - 10 Days. This self-drive itinerary covers the highlights of New Zealand's North Island, from Auckland to Wellington. Along the way, park your car and switch to your mountain bike to ride beautiful and interesting cycle trails.