Big Reef lies about 1.2 km south of Point Danger, the headland with the mainland gannet colony that guards the southern side of Portland Bay, and it sits directly on the boat route from the harbour to Lawrence Rocks. The Scuba Doctor observes that it can easily be overlooked on the way to somewhere else, but that it offers a vast array of interesting dive opportunities. The eastern side drops steeply from about 5 m on the reef top to 15 m on the sand, forming a low wall with ledges and undercuts; the western side descends more gradually in a series of ridges and gutters. The reef was once an important commercial abalone ground, which tells you about the habitat – clean rock, heavy algal growth and plenty of crevices – and although the commercial fishing has moved on, the reef still carries the western Victorian community in good numbers: kelp and mixed algae on the tops, sponges and ascidians on the eastern face, crayfish and abalone in the ledges, and blue-throat wrasse, magpie perch, sea sweep, herring cale, zebrafish, leatherjackets and boarfish over and under the structure, with rays on the sand and seals from Lawrence Rocks and Point Danger passing through.

It is an easy Open Water boat dive fifteen minutes from the harbour, with a wall to follow and a reef top to cross, and it makes a sensible first or second dive of a Portland day, a fallback when Lawrence Rocks is too rough, or a shallow finish after the Emily S. The Passage Bommie 160 m to the north and Little Reef nearby extend the same ground. Like everything at Portland it is very weather dependent and needs a low swell and light winds, and since no dive charter operates from the port it is dived from private boats and by arrangement with fishing charters, with Portland SCUBA the local source of air and advice. Victorian bag limits apply to crayfish and abalone.

Boats launch at the Portland Harbour ramps on Lee Breakwater Road (North Ramp and South Ramp), both with trailer parking, inside the harbour breakwaters. Run south out of the harbour and past Point Danger for about 5 km, roughly 15 minutes, to the reef 1.2 km south of the point; The Passage Bommie is 160 m to the north-north-west and Little Reef nearby. Anchor on sand off the steep eastern side.

Portland SCUBA in town supplies air; no dive charter operates from Portland.

The Scuba Doctor describes Big Reef as very weather dependent, best dived in good conditions with a low swell and light winds. It lies off Point Danger in Portland Bay with some shelter from the west but open to the south and east, so a south-west swell over about 1.2 m or any easterly sea makes it surge and clouds the water. Light northerly and westerly winds are ideal.

The Bonney Upwelling can bring cold green water in summer; autumn is clearest. Point Danger tide and swell forecasts apply.

Anchor on sand off the eastern drop in 14–15 m, descend and follow the steep eastern face along its length at 10–15 m, then cross over the top of the reef and return along the gentler western slope in 5–10 m. 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.