The peak is December to February, when the beaches are warm, the water is swimmable, and almost every bed in every hut is booked out. Huts must be reserved through DOC months in advance for this window. The pay-off is genuine summer weather — Tasman Bay is one of the sunniest corners of New Zealand.
November and March are the sweet spots: same long daylight, almost the same water temperature, but a fraction of the visitors. April and May give clear, cool weather and quiet beaches, though the bush is dripping after the first autumn rains. Winter (June–August) is genuinely quiet — frost on the early-morning grass, almost no-one on the track — and the huts are easy to book. Some water-taxi operators run a reduced winter schedule; check before relying on a pick-up.