The Sir Joseph Banks Group is a scatter of 17 islands at the southern end of Spencer Gulf, about 25 km off Tumby Bay, named by Matthew Flinders in 1802. The Marine Life Society describes clear water with good fish life in 10-20 m, and notes that larger charter vessels from Tumby Bay serve the group. Spilsby Island, at 468 hectares, is the largest, with a jetty on its sheltered west side, a 1 km beach and rocky dive sites on the south-east side.

The Society describes it as a major charter centre with diver accommodation. Fish life on the reefs is rich: the Society lists silver drummer, trevally, yellowtail, bullseyes, cowfish, blue groper, leatherjackets, wrasse, boarfish, zebrafish, sweep and morwong. The boulders and ledges carry sponges and algae, and the clear water of the outer gulf makes for good visibility.

Nearby Duffield Island (7.9 ha, 750 m from Spilsby) offers a 10 m dive with plenty of fish including large blue groper. The islands are part of the Sir Joseph Banks Group Conservation Park and marine park, and several host seabird and sea lion colonies – Langton Island's sea lions are one of the group's highlights. Spilsby suits divers with a capable boat or a charter who want to explore a little-dived island group.

It is a good base for a few days of island diving in calm summer weather. Carry a VHF radio or satellite beacon, full oxygen and first-aid kits, and enough fuel for a detour, as the group is a long way from help and weather in lower Spencer Gulf can change quickly. Check the marine park zoning map before fishing or collecting, and respect landing restrictions on the smaller islands, which protect seabird and sea lion breeding sites.

Tumby Bay is about 600 km (6.5 hours) from Adelaide via Port Augusta and the Lincoln Highway, or 50 km north of Port Lincoln (airport). Launch at the Tumby Bay ramp or join a charter and cross about 25 km south-east to Spilsby Island (45-60 minutes). The Marine Life Society notes larger charter vessels operate from Tumby Bay and that diver accommodation has been available on the island.

Anchor off the west-side jetty in the lee or on the south-east side for the rocky reefs.

The Sir Joseph Banks Group lies in open water in lower Spencer Gulf, 15-30 km off Tumby Bay. The islands give shelter from one direction or another, but the crossing is exposed to wind from all quarters, and strong south-easterly or south-westerly winds make it rough and unsafe for small boats. Tidal streams run between the islands.

Choose settled days with winds under about 15 knots; summer and autumn mornings are most reliable. Visibility is often good (10-20 m) in the clearer water around the outer islands.

On the south-east side, drop onto the rocky reef at 12-15 m and work along the boulders and ledges, then finish in the shallows; or snorkel near the west-side jetty.

Occasional charters from Tumby Bay (historically a major charter centre with island accommodation); private boats. Check current charter availability locally.