HMAS Bayonet (P 101) was an Attack-class patrol boat, one of twenty built for the Royal Australian Navy in the late 1960s for coastal patrol and, later, fisheries and immigration work in northern waters. She was laid down by Walkers Limited at Maryborough, Queensland, in October 1968, launched on 6 November 1968 and commissioned on 22 February 1969: 32.8 m long, 146 tons full load, two diesels for 21 knots, a 40 mm Bofors forward and a crew of 19. On 27 March 1982 she was transferred to the Melbourne Port Division of the RAN Reserve for reservist and cadet training out of HMAS Lonsdale and Western Port, and she paid off on 26 June 1988.

The Navy's original plan was to sink her in about 30 m as a recreational dive site; that fell through, and on 21 September 1999 Defence Maritime Services staff scuttled her instead in deep water off Cape Schanck. The wreck was located on 23 December 2001 by the Red October Group, a Melbourne technical team, after a year of planning. She sits upright on a sand bottom at 82 m with the deck at about 70 m, in strong current that has carved steep sand ripples around the hull.

Divers report the hull surprisingly clean, with the painted waterline markings on the bow still readable, a hole in the foredeck where the Bofors was removed, the wheelhouse and superstructure intact, and a mass of wiring visible inside – she was stripped of little. Twenty-five years on, sponge and sea-whip growth is established and ling, boarfish and crayfish have moved in. Bayonet is a trimix or rebreather expedition dive – the Scuba Doctor notes four cylinders per diver on open circuit – with 90 minutes or more of decompression in open Bass Strait, and it is attempted a couple of times a year at most.

It suits the most experienced Victorian technical divers, and it is a curiosity: the only modern warship wreck in the state apart from the Canberra, and a reminder of what the Canberra might have been had the Navy's first plan gone the same way.

Expedition only. The Bayonet lies about 45 km south of The Heads in open Bass Strait – 1.5–2 hours each way in a fast technical charter from Queenscliff Harbour or Portsea, or a similar run from Flinders (Western Port). Drive Melbourne–Queenscliff via the Geelong Ring Road and Bellarine Highway (1 h 40 min) or Melbourne–Flinders via the Mornington Peninsula Freeway and Boneo Road (1 h 30 min).

Trips are organised privately by Melbourne rebreather teams chartering a suitable vessel; nothing marks the site, the published position is a GPS mark from the 2001 discovery, and a good sounder is needed to find a 33 m hull in 82 m.

Open Bass Strait 30 km south of Cape Schanck with no shelter from any direction. The Scuba Doctor notes steep sand ripples around the hull from strong currents, and the site needs slack water as well as a swell under about 1 m and light winds for the whole day – a combination that occurs a few times a year, mostly in autumn. The 90-minute decompression is drifted under a float with the boat following.

Fog and the long passage home are additional factors. Visibility at 80 m in clean offshore water is often 20–30 m.

Expedition CCR dive: descend the shot to the deck at about 70 m, spend 20 minutes on the upright 33 m patrol boat – bow with its readable waterline marks and the empty gun mount, wheelhouse, stern – without entering the wiring-choked interior, then leave the shot on a DSMB and complete 90–120 minutes of drifting decompression with support divers at the shallow stops.

Expedition CCR dive: diluent around 10/60–10/70, full bailout for an 82 m ascent, oxygen and EAN50 deco, drifting decompression under a float with support divers and a live boat. Open circuit requires four or more cylinders per diver. Not a historic shipwreck under heritage law but treat as such.

No operator. Found and first dived by the Red October Group on 23 December 2001 after twelve months of planning; since then visited only by occasional Melbourne closed-circuit rebreather expeditions – a dive or two a year at most.