Duncan is on Vancouver Island, about 60 km north of Victoria and 60 km south of Nanaimo on the Trans-Canada Highway (Highway 1). Reach it from the mainland via BC Ferries from Tsawwassen to Swartz Bay (near Victoria), or from Horseshoe Bay to Departure Bay (near Nanaimo). The crossing takes 95 minutes and ferries run frequently, though summer waits can be long.
There is no scheduled air service to Duncan. The closest airports are Victoria International (YYJ), about 50 km south, and Nanaimo Airport (YCD), about 50 km north. A car is by far the most convenient way to get around once you arrive, particularly for the winery circuit.