The Nelson Bays region is the sunniest in New Zealand, which makes it appealing year-round, but the seasons matter for what you can do. December to February is peak summer — Abel Tasman is packed, Great Walk huts must be booked months ahead, and the beaches are warm. March and April are excellent: the summer crowds have thinned, the days are still long and warm, and the deciduous trees in Nelson's parks are turning gold.
Winter (June to August) is mild by New Zealand standards — rarely cold enough for frost in the lowlands — and the national parks are much quieter. The Heaphy Track is particularly good in winter since the West Coast section, often rainy in summer, tends to get clearer weather. The mountain bike trail on the Heaphy is also open to cyclists from May to November when it is closed to walkers. Nelson's arts scene operates year-round and is not season-dependent.