The J-class were the fastest submarines in the world when they were laid down for the Royal Navy in 1915: 84 m long, 1,210 t surfaced and 1,820 t submerged, driven by three diesels for 19 knots on the surface. Six survivors of the class were gifted to the Royal Australian Navy in March 1919, sailed out with the depot ship HMAS Platypus, arrived in Sydney on 15 July 1919 and were based at Geelong from 1920. Post-war budget cuts saw them paid off at Western Port on 12 July 1922, and in January 1924 the Melbourne Salvage Company bought the flotilla for scrap.

J4 launched at Portsmouth on 2 February 1916, commissioned on 17 July 1916 and transferred to the RAN on 25 March 1919. After sale to the Melbourne Salvage Company in January 1924 she sank by accident alongside at Williamstown on 10 July 1924, was refloated on 6 December 1926 and finally scuttled four miles outside the Heads on 28 April 1927 – the last of the four to go. The Melbourne Bottom Scratchers relocated her and fixed a bronze plaque to the conning tower, rediscovered on 16 May 1982.

The '26 m sub' lies on its keel running north–south with the bow pointing seaward. The bow section broke off during scuttling, so the four torpedo tubes and the modified bow structure are laid open and can be examined without any penetration. The conning tower is intact and in excellent condition, the best of any of the boats, and is usually wrapped in a cloud of fish that makes it the classic photograph.

Aft, the casing is open in places and the hull slowly merges into the limestone reef at the stern. Inside, the bulkheads that once held the boat together at depth and the engine bed are visible through the hatch openings. Full-length penetration is possible through multiple hatches, but the wreck is exposed to surge and the Scuba Doctor's warning is blunt: unwary divers can be sucked in and catapulted through the interior.

That same surge keeps the interior remarkably free of silt and the exterior scoured and colourful – on a clear day with 20 m visibility the whole 84 m hull can be taken in from mid-water, and it rates as one of Melbourne's top dives. At 19–28 m it is within Advanced Open Water limits with generous bottom time, which is why it is the most dived wreck outside Port Phillip and the usual first Graveyard dive for anyone progressing from the bay sites.

Boat only. Charters leave from Queenscliff Harbour (Red Boats, Dive Victoria Group) or Portsea Pier (Dive Victoria Group, Bayplay); drive Melbourne–Queenscliff via the Geelong Ring Road and Bellarine Highway (about 1 h 40 min) or Melbourne–Portsea down the Mornington Peninsula Freeway (about 1 h 45 min). Private boats launch at Queenscliff (Larkin Parade) or Sorrento ramps; the wreck lies a few kilometres outside The Rip, so time the run through the Heads for slack or a following tide.

Boat time from Queenscliff is roughly 20–30 minutes and from Portsea 30–40 minutes in a fast RIB, longer in a displacement charter boat. Gear is loaded at the harbour; there is parking at Queenscliff Harbour and along Point Nepean Road at Portsea.

Fully exposed Bass Strait. South-westerly swell is the killer: most charters will not run on a forecast over about 1.5 m, and anything from the south or west over 2 m makes the site undiveable and the passage through The Rip dangerous. Northerly and north-easterly winds flatten the sea; a calm morning after several settled days gives the best visibility (often 15–25 m in autumn and winter).

Surge is felt on the wreck in any swell over 1 m. Tidal streams outside The Heads are weaker than inside but still run; charters aim for slack water at Port Phillip Heads or a favourable tide for the transit. Winter westerlies close the site for weeks at a time.

The wreck sits in or near the shipping approach and pilot boarding ground, so a shot line, surface cover and a delayed SMB are mandatory.

Shot is dropped on the conning tower (about 21 m). Descend, check the bronze plaque on the tower, then swim north along the deck to the broken bow where the four torpedo tubes and bow modifications are open to view (26–27 m on the sand). Return along the opposite side of the casing, peer through the open hatches without entering unless trained and lined, and finish at the stern where the hull merges into reef.

Back to the shot for a 30-minute dive; safety stop on the line at 5 m.

Recreational. Nitrox EAN32 extends bottom time usefully at 25 m. Penetration divers use a reel from the conning tower hatch; surge can reverse direction mid-passage.

Dive Victoria Group (Portsea/Queenscliff) – on the charter schedule most weekends year-round and midweek in summer; Red Boats (Queenscliff) – regular charters; Bayplay (Portsea) – advanced/deep course dives. The most frequently dived wreck outside The Heads; also used for Advanced Open Water and Wreck specialty training.