The SS Catterthun is one of the most historically significant shipwrecks on the NSW coast, an 1881-built, 2,179-ton steamer of the Eastern and Australian Steamship Company that plied the Australia-China route carrying gold sovereigns out and tea back. On 8 August 1895, having left Sydney bound for Hong Kong, she ran into a gale just after midnight near Port Stephens and subsequently struck a reef near Seal Rocks; 54 of those aboard were lost, with one lifeboat carrying 26 survivors managing to reach shore in what remains one of the deadlier maritime disasters on this stretch of coast. The ship was carrying between 8,000 and 11,000 gold sovereigns at the time of her loss, and while much of that treasure was subsequently recovered by salvage divers, around 1,000 coins are believed to remain unaccounted for — with long-standing local suspicion that some were quietly retained by early dive operators working the wreck rather than declared.

The wreck today lies in around 55 to 60 metres of water, placing it firmly in technical diving territory beyond recreational limits, with her steam engine and other heavy machinery the most substantial and recognisable remaining structure after more than a century of decay and the initial salvage efforts. It remains a popular dive for technically qualified divers specifically drawn by the combination of genuine depth challenge and the wreck's dramatic gold-and-disaster history, and it is usually approached as a dedicated technical charter rather than a casual addition to a standard Seal Rocks trip. Divers attempting it need trimix training, planned staged decompression and reliable surface support, and should treat the site with the same seriousness as any deep historic wreck given both the technical demands and its status as a protected historical site.

Boats for the Seal Rocks sites either run the longer distance down from Forster/Tuncurry (about 45-60 minutes) or launch locally from the small boat ramp at Seal Rocks village itself, near Sugarloaf Point, cutting the run to Big and Little Seal Rock down to 10-20 minutes. Seal Rocks is a small, remote settlement reached via Lakes Way/Mungo Brush Road through Myall Lakes National Park; there is no dedicated dive shop in the village itself, so most Seal Rocks charter diving is booked through Forster Dive Centre, with divers sometimes meeting the boat locally rather than travelling from Forster.

Seal Rocks sits on an exposed headland with Sugarloaf Point taking the full force of easterly and south-easterly swell — trips are cancelled in anything beyond about 1.5 m swell, and the site is often diveable when more sheltered locations further north are blown out, or vice versa, so conditions should always be checked specifically for Seal Rocks rather than assumed from Forster. Visibility is typically 15-30 m and can be excellent in settled autumn and winter weather; it drops after the Myall Lakes and Wallis Lake systems discharge following heavy rain. Grey nurse sharks are present at almost any time of year here, with numbers varying seasonally, and the wrecks (Satara, Catterthun) require calm, low-current conditions for safe deep/technical diving.

Descend the shot line directly onto the wreck's steam engine and machinery, the most substantial remaining structure, and work a short, planned circuit given the depth before beginning a long staged decompression ascent with dedicated surface support.

Trimix required given the depth; plan staged decompression with multiple gases and a drifting or fixed deco station; not a dive for anyone without genuine technical training and experience.

Occasional technical charters via Forster Dive Centre or private technical dive clubs for suitably trimix-qualified divers; not a standard recreational-schedule site given the depth.