December to April is the busy season — Tasmanian and mainland summer holidays, the warmest sea temperatures (still only 17–19°C — wetsuits help), and the most reliable weather. January is the busiest month; if you can travel in late February or March you'll get similar weather with fewer crowds.
Autumn (March–May) is excellent for mountain biking at Blue Derby — cool, dry, no flies — and the rainforest at Pyengana is at its lushest after summer rain.
Winter (June–August) is quiet, often crisp and sunny in this rain-shadowed corner of Tasmania, and good for serious bushwalkers and surfers. Many tourist businesses close mid-week.
The St Helens Game Fishing Classic is in late February; the Suncoast Jazz Festival in late August fills the town for a weekend.