Thunder and Lightning Bay Reef is a small central rock surrounded by moderate granite reef in about 10 m, rated 6 out of 10 by the Marine Life Network guide as a popular stop on the return from Cambridgeshire dives. The rock supports colourful invertebrates and an attractive seaweed garden, and clean ocean currents make for good photography. The guide describes it as excellent crayfish habitat, with 3.5-4.5 kg animals common, and warns that the biggest are hard to catch and can be aggressive. For charter divers under supervision it rates the dive as straightforward. Local advice from Flinders Island Dive or the charter skippers at Lady Barron on current conditions, tides and exact positions is the best starting point for any dive here.

The Furneaux Group - Flinders, Cape Barren, Clarke, Preservation and dozens of smaller islands - is one of Australia's least-dived temperate regions. Granite dominates, but Flinders Island also has unusual limestone reefs, and the islands' position on the eastern approach to Bass Strait left a long trail of shipwrecks from the colony's earliest years. The Marine Life Network guide remains the fullest published account of the dive sites; since then Flinders Island Dive at Emita has made air and guiding available on the island.

All wrecks over 75 years old are protected under Commonwealth and Tasmanian heritage law: nothing may be disturbed or removed, and new finds should be reported to the Parks and Wildlife Service. There is no chamber on the island, so a diving injury means aeromedical evacuation to Hobart or Melbourne; dive conservatively and carry oxygen.

Flinders Island is reached by air from Launceston (about 35 minutes) or Melbourne, landing at Whitemark, or by the vehicular freight ferry from Bridport. Flinders Island Dive at Emita can supply cylinders and weights, which avoids air-freighting heavy gear. Whitemark's ramp is difficult at tide extremes; Lady Barron in the south has the best ramp and most sheltered anchorage and is the base for trips to Cape Barren, Preservation and Clarke islands.

Thunder and Lightning Bay Reef is usually dived by charter boat on the way back from the Cambridgeshire wreck, south of Cape Barren Island; allow over an hour from Lady Barron.

The Furneaux Group sits in eastern Bass Strait: the west coast of Flinders Island is exposed to the prevailing westerlies, the east coast to easterly summer gales, and strong tidal streams run through Franklin Sound, Armstrong Channel and the passages between the islands, especially after gales. January-April offers the most settled weather, but sudden southerly or westerly changes can arrive with little warning, so plan around a safe anchorage. Clean ocean currents flow over the reef.

Drop near the central rock, circle it looking at the invertebrates and seaweed garden, then explore the surrounding granite reef. Turn the dive at half gas (thirds where current or overhead sections are involved), deploy an SMB before surfacing for the live-boat pick-up, and complete a 3-minute safety stop at about 5 m where depth allows.

Flinders Island Dive (Emita, west coast; ph 0428 598 529) offers scuba and snorkel hire, air, and small-group guided diving, snorkelling and fishing tours including reefs, caves and shipwrecks, year-round. Charter boats Rockjaw Tours and Latitude Charters (listed by Visit Flinders Island) run trips that can include diving; the southern wrecks are reached from Lady Barron.