Lonsdale Rock is a kelp-covered rock outcrop at the western side of the Port Phillip entrance, a few hundred metres off Point Lonsdale lighthouse and only 570 m south of 16lb Reef on the inner Lonsdale wall. The Scuba Doctor listing gives a depth range of 6–15 m and an Open Water rating, and its one descriptive note is a navigational one: the rock's position 'is discernible by the oily appearance of the water due to kelp growing on it' – the slick that a canopy of bull kelp and Ecklonia makes on the surface when the water is calm.

That detail says most of what a diver needs to know. This is a shallow, kelp-dominated pinnacle in high-energy water at the very mouth of The Rip, the kind of habitat that on this coast holds southern rock lobster, abalone, big bluethroat wrasse, sweep and, around the fringes, weedy seadragons and the encrusting life that colonises every hard surface at the Heads. The rock is surrounded by broken limestone reef in 12–15 m running toward the Point Lonsdale platform and the wrecks of the Lonsdale reef, and toward 16lb Reef and the start of the wall to the north.

It is not a charter site. No tide window is published, but its position leaves no doubt that it can only be dived at slack water – the ebb slack, like the rest of the outer Lonsdale reef – in calm conditions, and that the flood would carry a diver straight onto the reef and into the entrance. Club and private boats dive it occasionally as a shallow, sheltered-from-depth alternative on a calm day, and it would suit a photographer wanting kelp-forest images close to the Heads. The description here is short because the published record is short; data confidence is accordingly moderate.

Boat access only. Charter boats leave from Portsea Pier (Dive Victoria Group, Bayplay) and Queenscliff Harbour (Dive Victoria Group, Red Boats); run time is about 15 minutes. Private boats launch at Sorrento Boat Ramp (Point Nepean Road, Sorrento, all-tide concrete ramp with trailer parking) or Queenscliff Boat Harbour ramp (Larkin Parade, Queenscliff), roughly 15 minutes to the site.

The site is just outside the western side of the entrance off Point Lonsdale; the listing notes the position 'is discernible by the oily appearance of the water due to kelp growing on it'. Skippers must call Lonsdale VTS (Point Lonsdale) or listen to VHF 12 for shipping movements before dropping divers, and must carry a dive flag; anchoring is not permitted in the shipping channel, so a live boat and a drop-shot or drift float is the normal method.

It is dived on the ebb slack or a gentle ebb only, when the outgoing tide leaves this side of the reef in relatively still water. Slack water at Port Phillip Heads does not coincide with high or low water: it falls roughly three hours after high or low water at Point Lonsdale, and the usable window is typically 20–40 minutes. Use the Ports Victoria / Port of Melbourne slack-water predictions for The Heads (also republished by charter operators and WillyWeather 'Rip Bank') rather than a standard tide table.

Being outside or right at the entrance, it is exposed to Bass Strait swell: anything over about 1.5 m from the south-west or south, or a strong southerly/westerly wind against the ebb, produces standing waves in The Rip and charters cancel. Northerlies flatten the entrance and give the best days. Visibility is best in late summer and autumn (Feb–May) when the water is clear and the westerlies have eased; winter brings 11–13 °C water and bigger swells but often the clearest days after a run of settled weather.

Heavy rain has little effect this close to the ocean. Very close to the Point Lonsdale rock platform and the Rip: the ebb runs hard past it and the flood sets onto the reef, and it breaks in any swell; a calm-day ebb slack only.

Drop up-current of the kelp slick at ebb slack, descend onto the rock top at 6 m and circle the base at 12–15 m looking into the ledges, then finish among the kelp on top. Safety stop on the rock; ascend under a DSMB clear of the kelp.

No regular scheduled charter – dived from private and club boats (VSAG, Power House Underwater Club, Scuba Diving Melbourne) and occasionally by Dive Victoria Group or Red Boats when a group requests it.