Rotomahana was a landmark ship. Built by William Denny and Brothers at Dumbarton in 1879 for the Union Steam Ship Company of New Zealand, she was the first ocean-going vessel built of mild steel and one of the first with bilge keels; 90.9 m long, 1,777 tons and fast enough to be called the 'Greyhound of the Pacific'. She ran the trans-Tasman route and later the Melbourne–Launceston Bass Strait service until the 1920s.

Sold for scrap, she was stripped and scuttled with a gelignite charge in the engine room on 29 May 1928. The wreck lies in 35–39 m with the bow roughly to the east and is badly broken: the scuttling charge and 90 years of Bass Strait swell have reduced most of the hull to plates, girders and debris spread across the sand. What remains is nonetheless impressive in scale.

Amidships stand four large condensers and the boilers, rising several metres and forming the core of the dive, with some pass-through possible between and beneath them for wreck-trained divers. The clipper bow has separated from the rest of the hull and lies apart with anchors, winches and the bowsprit still in place – the most photogenic corner of the site. A deckhouse that was once penetrable has collapsed.

Large schools of butterfly perch, bullseyes and sweep patrol the boilers, with ling and crayfish in the debris and sponges and zoanthids on every upright surface. Visibility here is often better than on the inshore subs because the site is further from the Barwon River plume. Because most of the interesting structure sits at 36–39 m, bottom time is short and every Rotomahana dive carries some decompression.

It suits experienced deep divers who value history as much as intact structure, and it is a popular first 'proper' technical wreck for Melbourne divers moving on to trimix.

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 on the boilers/condensers amidships at about 36 m. Work east along the debris field to the separated clipper bow with its anchors, winches and bowsprit, then back past the boilers (some pass-through between and beneath them for trained divers) toward the stern. With only 18–20 minutes at 37–39 m stay on the main mass; return to the shot and ascend with 10–15 minutes of stops.

Within recreational limits on air or EAN28–32 but an SMB, redundant gas (pony or twinset) and a computed deco plan are expected; many divers run it as a light-deco dive on nitrox with a 50% stage. Shot line descents and ascents only – free ascents in the shipping approach are not acceptable. Many divers run EAN28 back gas with EAN50 deco for 20 minutes on the boilers.

Dive Victoria Group (Portsea and Queenscliff) – scheduled double-dive charters most weekends in season, weather permitting; Red Boats (Queenscliff) – regular weekend and weekday charters; Bay City Charters (Portsea/Queenscliff) and Chimney Rock Charters (Queenscliff) – regular weekend charters; Bayplay (Portsea) – occasional advanced charters. All treat the site as a deep/advanced dive and run it on the calmest days of the week. Less frequent than J4/J5/Coogee because of the 39 m sand depth; typically two or three charter visits a month in good weather.