Flat Rock is a low shore platform on the eastern side of Cape Nelson about 9 km south-west of Portland, and the reef off it is, in the Scuba Doctor's words, a very worthwhile boat dive with swim-throughs, drop-offs and ledges in the shallow areas and rock walls extending outward to about 24 m, with the best diving between 5 and 18 m. The eastern side of the cape is in the lee of the prevailing westerlies, so Flat Rock is one of the Cape Nelson sites that can be dived when the cape itself and the western side are unworkable, and it is correspondingly the most visited. The basalt reef carries kelp on the shallow tops and the heavy encrusting growth of sponges, ascidians and bryozoans on the walls and in the swim-throughs, and the fish life is described as prolific: blue-throat wrasse, magpie perch, sea sweep, herring cale, zebrafish, leatherjackets, banded morwong and boarfish, with catsharks and rays on the sand and cuttlefish and octopus in the ledges. Black-lip abalone and southern rock lobster are common enough that the site guide notes both may be taken in season with a catch bag, subject to Victorian limits, and seals from Cape Bridgewater pass along the coast.

With a depth range from 5 to 24 m it suits everyone from Open Water divers, who can stay in the shallow swim-through zone, to more experienced divers who want the walls, and the anchor-on-sand layout keeps navigation simple. It is 20–25 minutes by boat from the Portland Harbour ramps, closer than the cape proper, and it combines with the Isabella wreck and the Cape Nelson lighthouse reefs further round for a full day when the weather allows. No dive charter operates from Portland, so access is by private boat or arrangement with a fishing charter; Portland SCUBA in town supplies air and local knowledge.

Boats launch at the Portland Harbour ramps on Lee Breakwater Road (North Ramp and South Ramp), both with trailer parking, inside the harbour breakwaters. From the harbour run south-west around Point Danger and along the eastern side of Cape Nelson for about 9 km, 20–25 minutes, to Flat Rock, an obvious low platform on the shore; anchor on sand off the reef. Killer Waves is 1.3 km to the south-south-west.

Nelson Bay tide and swell forecasts apply. Portland SCUBA in town supplies air; no dive charter operates from Portland.

Flat Rock is on the eastern side of Cape Nelson and is sheltered from westerly and south-westerly winds by the cape, which makes it the Portland-side reef to choose when a westerly is blowing; it is open to the south and east, so southerly swell over about 1.2 m or easterly winds make it surge and cloud the water. The Scuba Doctor's advice is good conditions with a low swell and light offshore winds. The Bonney Upwelling brings cold green water at times in summer; autumn is clearest.

Nelson Bay forecasts apply.

Anchor on sand in 12–15 m, descend and follow the rock walls seaward to 18–24 m if visibility and gas allow, then return along the wall and spend the second half of the dive in the 5–12 m zone working through the swim-throughs, drop-offs and ledges near the shore. Turn at half gas, return to the anchor and ascend with a 3-minute stop.

No dedicated dive charter operates from Portland. Portland SCUBA (PADI centre) runs courses and occasional day trips; local fishing charters take divers by arrangement; Portland dive club members use private boats. Warrnambool Diving visits occasionally.