Every winter the rocky coast north-east of Whyalla hosts one of the world's great marine wildlife spectacles. Giant Australian cuttlefish (Sepia apama), the largest cuttlefish species – males grow to about 50 cm and 5 kg – migrate into the shallow reefs between Stony Point, Point Lowly and Black Point to breed. The aggregation is the only known mass spawning of the species anywhere, and the national heritage listing records numbers fluctuating from a low of 13,492 in 2013 to a peak of 247,146 in 2020.

The attraction is the habitat. The shore is a fringing reef of fragmented bedrock slabs, giving thousands of crevices and overhangs where females attach their eggs, and the cold winter water (about 12°C) is needed for incubation. Males outnumber females, and the reef becomes a stage of competition: big males flash rippling bands of colour, extend their arms and spar with rivals, while smaller 'sneaker' males mimic females to slip past the guards.

It all happens in 1-6 m, within a few metres of shore, and the animals are indifferent to snorkellers and divers who stay calm and still. Stony Point is the main access point, with a boardwalk, handrail, change rooms and toilets. Snorkellers see nearly everything in 1-2 m, while divers can settle on the bottom in 4-6 m and watch courtship at eye level; photographers often spend hours in the water over several days.

The aggregation nearly collapsed in the early 2010s, prompting fishing closures, the sanctuary zone and heritage listing, and numbers have since recovered. Visiting requires a thick wetsuit or drysuit, patience with the cold, and respect for the animals. For anyone diving in Australia in winter, it is a must-do: an encounter with tens of thousands of intelligent, colour-changing animals in water shallow enough to stand up in.

Whyalla is about 390 km (4-4.5 hours) from Adelaide via Port Wakefield, Port Augusta and the Lincoln Highway, or a 45-minute flight. From Whyalla take the Lincoln Highway north, then Port Bonython Road east towards Point Lowly; Stony Point is signposted on the right about 30 minutes (roughly 25 km) from town, just before the Port Bonython facility. There is a large car park, change rooms, toilets and a shelter, and a boardwalk with a chain handrail leads to the water.

The rocks at the water's edge are uneven and slippery with weed – wear boots and enter carefully. Cuttlefish are often in water under 2 m within a few metres of the entry.

The aggregation runs in winter, when the water is 11-15°C and mornings are often still and clear. Strong southerly or south-westerly winds push chop onto the Stony Point shore and stir the shallows, reducing visibility and making the rocky entry hazardous; northerly and north-westerly winds blow offshore and give the flattest water. Tidal state matters: an incoming tide is said to push cuttlefish closer to shore, and visibility is best around slack water (often 20 m+), while a running tide can make it murky.

Morning sessions usually have less wind and clearer water. There is no swell.

Enter at the boardwalk, then drift slowly along the shallow ledges parallel to shore in 2-6 m, looking under overhangs for females laying eggs and watching males display. Stay low and still – the animals approach to within arm's length. Turn at half gas or when cold and return along the same ledge line; snorkellers do the same on the surface.

Shallow shore site; drysuit or 7 mm with hood recommended for long photography dives. Weight belts help snorkellers stay down for photos.

During the season: Whyalla Dive Shop (Why Dive) runs daily guided snorkels and dives with wetsuit/gear hire; Diving Adelaide runs multi-day cuttlefish trips from Adelaide (often with Tumby Bay jetty); local guide Tony Bramley pioneered cuttlefish tourism and guided tours. Many visitors go independently.