What is New Zealand's most popular travel destination? Is December good time to go?
December is a good time. Weather-wise, November to April is usually pretty good and January, February and March can be hot (by northern European standards!). But the whole of the country has a maritime climate so the weather can change at any time and, if there's a southerly from the Antarctic, it can be chilly for a while. In the South Island and in the centre of the North Island, there are ski-fields (it was snowing in Christchurch last week).
As to the most popular destination, it depends what you want. The South Island is unbelievably scenic - Milford Sound, the Southern Alps, Queenstown and the lakes, all the national parks - definitely worth spending a lot of time there. Some people diss Auckland (built on 49 dormant volcanoes) but that's a great city in which to live - busy but with easily accessible scenic parks and beaches. In the North Island, as well as Auckland, Wellington, Matamata (Hobbiton), Tauranga and the Bay of Islands are all tourist destinations.
Have a look at our travel guide: https://www.travellerspoint.com/guide/New_Zealand/
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December is normally peak tourist season in the honeypot locations like Queenstown, but I'm reading that international tourism is still at a fraction of pre-pandemic levels, so this may be a great time to go.
The weather in December can still be quite mixed, with some rainy spells, before it settles down in January for more summer weather. Nothing that would stop me going though.
As Boris says, the Queenstown area is the hub of the scenic area of mountains and lakes. I'd suggest planning some time based in each of Queenstown, Te Anau, and Wanaka, with maybe a side-trip to the west coast glaciers at Fox and Franz Josef.
How long have you got, and what do you like doing? If you're more interested in the geothermal areas - geysers, volcanoes, fumaroles, hot springs - then look at the area around Rotorua / Taupo / Tongariro.