Partridge Island lies off the south-western coast of Bruny Island at the southern entrance to the D'Entrecasteaux Channel. The Marine Life Network guide rates it highly: the northern side is sheltered from the prevailing southerly swell, with thick weed, a tall Macrocystis forest on the channel side and depth increasing progressively seaward, in 2-10 m. It records reef fish and green seaweeds, notes that the island is heavily fished, and describes it as good for groups and for camera work.

The tall giant kelp is the highlight, though – as everywhere in south-east Tasmania – its condition now varies from year to year with water temperature and may be much reduced from the forest described. Beneath the canopy, wrasse, leatherjackets, bastard trumpeter and magpie morwong feed, and seahorses and weedy seadragons may be found. It is an easy, shallow dive for all levels once there, but reaching it requires a boat and a crossing either from Bruny Island or from the Dover–Southport side.

D'Entrecasteaux's expedition anchored in these waters in 1792-93, and the channel and the island names reflect French and early colonial charting. Ask locally about the current state of the kelp before making the trip. With good planning it makes a rewarding day trip combined with Quiet Bay or Courts Island, and the scenery of the southern channel entrance – with Bruny's cliffs and the far south's mountains – is outstanding.

Carry a VHF radio and let someone know your plans, as mobile coverage is patchy. Snorkellers can enjoy the canopy from the surface on calm days.

Bruny Island is reached by the vehicle ferry from Kettering (about 35 km, 40 minutes south of Hobart via the Southern Outlet and Channel Highway) to Roberts Point on North Bruny; the crossing takes about 20 minutes and runs frequently, but book or queue early in summer. Launch at Lunawanna/Cloudy Bay area ramps on South Bruny or cross the channel from Dover/Southport; Partridge Island lies off Bruny's south-western shore. Anchor on the sheltered northern side in sand, clear of the kelp.

Bring filled cylinders; there are no fills on Bruny.

The D'Entrecasteaux Channel is sheltered from ocean swell by Bruny Island, but strong northerly and southerly winds funnel along it and raise a steep chop. Southerly swell entering from the Southern Ocean affects the southern channel sites. Visibility is moderate and often green-tinged from Huon River and local runoff, dropping after heavy rain; the Marine Life Network notes a possible slow tidal current at some sites.

Calm mornings in late summer and autumn are best; the channel can be dived year-round. The northern side of the island is sheltered from the prevailing southerly swell.

Descend on the northern side into the weed, move to the channel side where the Macrocystis grows, and swim among the stipes as the bottom deepens seaward. Return to the anchor through the weed.

Underwater Adventures Tasmania – Channel and Bruny boat trips from Hobart; otherwise private boats from channel ramps (Kettering, Woodbridge, Middleton, Gordon, Alonnah).