Lady Julia Percy Island – Deen Maar to the Gunditjmara, for whom it is a place where the spirits of the dead depart – is a flat-topped volcanic island of about 130 hectares lying 8 km off the coast between Port Fairy and Portland. Its 30–40 m basalt cliffs are undercut with sea caves and drop into boulder reefs and sand in 15–25 m, and around its base lives the largest breeding colony of Australian fur seals in Victoria, several thousand animals that pup in December and are present all year. It is the wildest big-animal dive in the state. The seals, especially the juveniles, are relentless in the water, circling, buzzing and mouthing fins; the basalt walls are coated in sponges, ascidians, bryozoans and hydroids in the colours that cold, upwelled Southern Ocean water produces; crayfish sit in every ledge; and schools of sweep, butterfly perch, boarfish and salmon move along the cliff base. The Scuba Doctor's general mark is on the sheltered western side, chosen from aerial imagery as the most likely lee, and the site guide is candid that the underwater terrain is spectacular all the way round the island and that information on specific spots is thin. The named sites that have been documented – Landing Bay on the north, the Blowhole cave and Leopard Seal Reef on the south – are recorded separately.

Everything about the island is serious. It is a long, exposed crossing from Port Fairy or the difficult Yambuk river entrance; there is no dive charter based nearby, so trips depend on fishing boats or private vessels; the Bonney Upwelling makes the water cold and sometimes green in summer; and great white sharks hunt the colony, most obviously during the summer pupping season. Landing is prohibited without a permit and the wildlife regulations on approaching seals apply. For a well-organised group with a good boat on an autumn high, however, a day at Deen Maar – a wall dive under the cliffs in the morning, a shallow seal dive in the afternoon – is the finest diving in south-west Victoria and comparable to anything in Bass Strait. It suits experienced divers comfortable with big animals, cold water and remote boat operations.

Boat only. From Port Fairy launch at the Griffiths Street ramp on the Moyne River and run 20 km west-south-west, 40–60 minutes in a fast boat; from the Yambuk ramp on the Eumeralla River (Lake Yambuk) the island is 8–10 km offshore but the river entrance is shallow and conditions-dependent; from Portland Harbour it is about 35 km east. Charter boats from Port Fairy or Portland take divers by arrangement.

Choose the side of the island in the lee of the swell; the western and northern sides are the usual anchorages. Landing on the island is prohibited without a permit.

Lady Julia Percy Island stands 8 km offshore in the open Southern Ocean and the crossing from Port Fairy or Yambuk is exposed in any weather; the island offers a lee on one side or the other but nowhere is calm in a swell over about 1.5 m. Plan for a south-west swell under 1–1.5 m and winds under 15 knots, choosing sites on the northern (Landing Bay) side in southerly weather and the southern side in northerlies. The Bonney Upwelling brings cold, nutrient-rich water from November to April, when visibility can drop and water temperatures fall to 13–14 °C even in summer; autumn generally gives the clearest water.

Great white sharks frequent the colony, particularly during the pupping season from November to January. Diveable perhaps 30–40 days a year for a boat based in Port Fairy.

Anchor on sand in the lee in 12–15 m, descend the line and work along the base of the basalt cliff, where boulders, ledges and cave mouths hold crayfish and the sponge growth is heaviest, letting the seals come to you in the shallower water above 10 m. Keep the wall on one side, turn at half gas, return to the anchor and complete a 3-minute stop watching the seals around the boat.

No regular charter. Port Fairy and Portland fishing charters take divers to the island by arrangement in settled weather; Warrnambool Diving and Melbourne clubs organise occasional trips; most dives are from private boats out of Port Fairy or the Yambuk (Eumeralla River) ramp.