New Zealand's seasons are the reverse of the Northern Hemisphere. Summer runs December to February, winter June to August. The climate varies significantly between north and south — the far north of the North Island is subtropical while the southern South Island gets proper winter cold and heavy snowfall in the mountains.
Summer (December–February) is peak season. School holidays in New Zealand and Australia fill up campsites, beaches, and the main tourist spots. Prices rise and the Tongariro Crossing, Milford Sound, and Queenstown get crowded. The weather is warm and reliable, days are long, and the country is at its most accessible. Book accommodation and ferry crossings well in advance.
Autumn (March–May) is genuinely excellent, especially in the South Island. The summer crowds have thinned, the weather stays warm, and Central Otago and Marlborough put on good colour. March is often cited as the best month for long-distance walks — conditions are settled and the worst of the summer rush is over.
Winter (June–August) is ski season. Queenstown and Wanaka in the South Island, and the Central Plateau (Ruapehu) in the North Island, are the main ski areas. Outside ski towns, winter is quiet and prices drop. Fiordland gets even more rain than usual in winter, which means the waterfalls at Milford Sound are at their most dramatic.
Spring (September–November) brings wildflowers, warming temperatures, and shoulder-season prices. Weather is changeable but conditions improve steadily through the season. October and November are increasingly popular as alternatives to the full-price summer rush.
In general: if you want good hiking and beach weather without peak-season crowds, aim for March or November. If skiing or snowboarding is the point, come in July or August. Summer delivers the most reliable weather and the longest days, but you'll pay more and share the highlights with many others.