Looking to stay here for 3 days to explore the areas of Russell, Keri Keri, do bike trail one day, bay of islands one day and forest walks. Would love suggestions please
Paihia is a town. Opua is more of a place where the ferry departs from. They aren't that far apart from one another. If you want to be close to where boat tours depart from, grocery stores, activity, then stay at Paihia. If you want to be away from tourists, then stay at Opua.
Personally, I'd stay in Russell.
Paihia is the tourism hub for the Bay of Islands. It is the centre for the bay cruises, the passenger ferry services to Russell, has more established accommodation, dining, and closer proximity to Haruru Falls, Te Tii beach, Waitangi and Te Haumi bay.
Opua is lovely, and has commanding views of the inlet and easy access to the vehicular ferry. The walking track from Opua to Paihia has reopened. Opua is the start of the biking trail to the Hokianga harbour, and also the rail attraction betweeen Opua and Kawakawa. Limited dining at the old store/cafe, and the marina cafe.
Go with available accommodation and what suits your plans best.