It actually depends where you are in the country. Westland and Fiordland have most rain in spring, Central Otago has most in summer, and Southland is quite even all year round.

NZ’s weather is dominated by the ocean, which mitigates the heating and cooling effects of large land masses, and by being in the mid-latitude westerlies. The band of westerly winds migrate further north in winter and spring, due to the differences in temperature between the tropics and Antarctica. The instability of the westerly flow brings a series of highs and lows.

But also we’re close enough to the subtropics that we’re affected by lows from the north. Since those lows have come across warm water, they pick up a lot of moisture.

Monsoons, on the other hand, are caused by large-scale heating across a continent e.g. Australia and south Asia. The centre of those continents can be ten to twenty degrees warmer than the surrounding ocean. That drags in wind across thousands of miles (due to the hot continental air rising). When the wind blows across the tropical oceans, it picks up a lot of water, which comes down as rain. Here in NZ, the summer heating is much smaller. We do get localised convective showers on hot days, but there isn’t enough land to affect the large-scale wind flow. The water around us does heat up a little in summer, but it’s small fry compared to the inland heating of Australia and Asia, and it isn’t enough to disrupt the westerly flow.

Phew…I hope all that makes sense.