Middle Island sits in the mouth of the Merri River on the western side of Stingray Bay, a low rock islet famous above water for the little penguin colony that was saved from foxes by maremma guardian dogs – the story behind the film Oddball. Below water it is the richest part of the Merri Marine Sanctuary. Between Middle Island and its neighbour Merri Island runs a deep canyon reaching about 15 m, and Parks Victoria's description of it – kelps swaying with the waves, colourful sponges beneath the overhangs – understates a slot packed with life. The Scuba Doctor describes boulders, channels, overhangs and swim-throughs, and lists Port Jackson sharks, gulf wobbegongs, stingrays, giant Australian cuttlefish, parrotfish (rainbow cale), zebrafish, southern rock lobster and abalone as the common residents; add the draughtboard sharks, eagle rays, seahorses and schooling sweep of Stingray Bay, and the fur seals and dolphins that come in from outside. From August to February little penguins and short-tailed shearwaters breed on the island and the penguins can be seen hunting in the sanctuary waters at dusk.

Two dives are possible. The sheltered option is to swim from Stingray Bay into the channel and dive the canyon, which is protected enough to be done on many low-swell days and is a straightforward Intermediate dive with a strong sense of place. In very calm conditions experienced divers continue around to the exposed Southern Ocean side of the island, where kelp forests and bigger fish face the open sea; this side, the Scuba Doctor warns, turns into dangerously rough water with big swells and strong currents when conditions deteriorate, and it should only be attempted after watching the sea for a while. Both dives start and finish at the Stingray Bay steps. The island itself is closed to the public and landing is prohibited; the sanctuary is no-take throughout. For most visiting divers the canyon is the single best dive in Warrnambool that can be done from shore.

Park on the western side of Viaduct Road near the concrete wall and take the steps to Stingray Bay beach (under a minute). Enter from the sand and swim west along the bay toward the channel between Middle Island and Merri Island, about 200 m; descend at the channel mouth. On very calm days experienced divers continue around to the exposed Southern Ocean side of the island.

Do not land on Middle Island – it is closed to the public to protect the penguin colony. Exit back at Stingray Bay. Warrnambool Diving on Fairy Street is 2 km away.

The Merri Marine Sanctuary sites face south-west into the Southern Ocean and are, in the Scuba Doctor's words, prone to surge and swell, with conditions able to change quickly into dangerously rough seas with big swells and strong currents. Dive at low tide with a low swell (under about 1 m at Stingray Bay, under 0.5 m for the exposed sites) and light offshore northerly to north-easterly winds; any onshore southerly through westerly wind makes the sites treacherous. Check the water from the car park – if you see lots of white water, head home.

Visibility is best after settled northerly weather in autumn and poor after rain from the Merri River. Warm-water upwelling is absent; the Bonney Upwelling can bring cold green water in summer. The channel between the islands is sheltered enough for the inner dive on a low swell, but the outer, Southern Ocean side of Middle Island is only for very calm conditions and experienced divers.

From Stingray Bay swim west and descend at the mouth of the channel between Middle and Merri Islands in 5–6 m. Follow the canyon through, dropping to its 15 m floor, exploring the boulders, overhangs and swim-throughs on both walls and the kelp on the rims, then return through the canyon rather than around the outside unless the sea is flat. Turn at half gas and surface in Stingray Bay well clear of the island.

Warrnambool Diving (Fairy Street) – air fills, PADI courses, divemaster-led local shore dives on request; Daktari Surf & Dive (Warrnambool) – snorkel gear and guided snorkels; Warrnambool dive club members dive the sanctuary regularly on calm days.