The Coromandel Peninsula is at its most appealing in late December through February — warm temperatures, long days, and water warm enough for swimming. The downside is that it is also at its most crowded; accommodation is booked months in advance over the Christmas/New Year period.
March and April are excellent: the crowds thin after February, the water is still warm, and the light softens into the appealing golden tones of late summer. This is the best time for Hot Water Beach, Cathedral Cove, and the forest walks.
Winter (June to August) brings cooler temperatures and frequent rain but also solitude. The forest walks are beautiful in the rain, the towns revert to their quiet selves, and accommodation is cheap and available.
The peninsula is only two hours from Auckland, so even in the main summer season midweek visits are significantly less crowded than weekends.