Coffs Harbour has a subtropical climate, with warm humid summers and mild dry winters. The town is comfortable year-round and is busiest in the December–January school holidays, when prices double and the beaches fill.
Spring (September–November) and autumn (March–May) are the sweet spots: warm enough to swim, dry enough to walk inland, and quiet between the school holidays. The shearwater colony on Muttonbird Island is most active from late September.
For whale-watching, time a visit for June–early July (humpbacks heading north to Queensland) or September–November (returning south with calves). The water at the marina runs deeper close to shore and the whales pass within a few hundred metres of the breakwater.
Avoid the December–January school holidays unless you have booked accommodation well in advance. The shoulder of late January through February is hot and humid but with much smaller crowds.