Noosa's high season runs from December to late January (Australian school holidays) and again over Easter. In those weeks the town is full, Hastings Street is bumper-to-bumper, and you need to book accommodation and restaurants well ahead.
The best months to visit are September to early December, and February to May. The water is still warm, the sun is reliable, and the crowds are manageable. Winter (June to August) is the surprising favourite of many locals: bright, dry, mid-20s by day, and the humpback whales pass close to the headland on their migration. Avoid school holidays if you can.