The AV Ulonga is the deepest and most challenging wreck in this region, a historic vessel lying in 37-39 m far out in the gulf west of Rapid Head. According to the Marine Life Society of SA, she was built in 1910 by Permewan, Wright and Co. at Moama, New South Wales, as the iron-hulled paddle steamer PS Ulonga, a Murray River trader. In 1948 she was rebuilt as a three-masted schooner with auxiliary twin screws, 110.7 ft long with a 22 ft beam, and worked the coastal trade carrying gypsum, wool and superphosphate between Port Adelaide and Kangaroo Island.

On 6 July 1976, sailing from American River back to Port Adelaide, she foundered at about 6 pm. Her crew of five were rescued by the SS Star Lily and transferred to a prawn boat. One theory is that her seams were sprung by the expanding gypsum cargo.

The wreck lay undiscovered until divers found it around the mid-1990s to 2001. She sits upright, oriented roughly north-east to south-west with her bow toward Kangaroo Island. Divers have reported crates of soft drink bottles, deck fittings and portholes, and a Chrysler Valiant that has slipped off the deck onto the seabed, while an enormous wobbegong has taken up residence in the wheelhouse.

Snapper and trevally school around the hull. The Marine Life Network notes the wreck is covered in anchors, fishing lines and heavy silt. This is a dive for technical divers only: at 37-39 m, no-decompression time on air is minimal, narcosis is significant, and currents are fierce.

With suitable gas, planning and boat support, it offers a rare, rarely visited historic wreck.

Boat access only. Launch at Wirrina Cove (Marina St Vincent) and run west-north-west for about 20 minutes to the GPS mark, roughly 26.5 km due west of Haycock Point or 16.5 km from Rapid Head. There are no moorings: shot the wreck and have a boat attendant follow divers, as current is strong.

This is an exposed open-water location that needs a settled forecast.

Divers describe the current here as nothing short of ferocious. Dive only at slack water on neap dodge tides, with a shot line and live-boat pickup. The site is exposed to wind from all directions; a calm day with light easterly or southerly winds is essential.

Visibility varies with the current and sediment. Summer to autumn gives the most settled weather windows.

Shot the wreck at slack water, descend the line directly to the deck at about 35 m, explore the wheelhouse and deck (the Valiant car lies alongside), and begin the ascent on the shot before the current returns. Plan decompression on the line or a drifting deco station with the boat following.

Technical dive: trimix or at least EANx deco gases recommended; plan deco on the shot or a drifting station; line cutters essential due to fishing line; live-boat pickup required.

No regular operator – private boats and technical diving groups only, occasionally Marine Life Society of SA members.