The Swim Through is one of the named sections of the Lonsdale Wall cliff, sitting between Spot G and North Wall Corner on the northern part of the system. The Scuba Doctor listing gives a depth range of 20–30 m and describes it simply as 'yet another awesome dive site on Lonsdale Wall'; the name comes from a cleft where a slab of the limestone face has split away from the main wall, leaving a short passage that a diver can pass through from the terrace to the open face.

The topography is that of the whole wall in miniature. The reef top lies at about 20 m, covered in short kelp, sponges and bryozoans, and the lip falls away in steps with a deep overhang behind each. The swim-through itself is not a cave in the technical sense – there is daylight at both ends and no silt – but it is dark enough inside to need a torch, and the walls of the passage are plastered with the yellow zoanthids, jewel anemones and orange sponges that characterise the Heads. Blue devils and southern rock lobster use the passage as shelter, and old wives and boarfish hang in the eddy at its mouth.

Because it lies in the 20–30 m band, this is one of the more comfortable deep sections of the wall for Advanced divers: there is a defined feature to visit, a natural turn-around point, and no need to go past 30 m to see the best of it. Operators pair it with Spot G, which is only 12 m away along the face, and a diver may well cover both on the same slack. As with every site on this wall, the entire dive happens inside the slack-water window and ends with a drift under a DSMB.

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 on the northern part of Lonsdale Wall, a few boat-lengths west of North Wall Corner. 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 only on slack water, either at the end of the flood (SWF) or the end of the ebb (SWE); most operators prefer the flood slack so that any residual current sets into the bay rather than out to sea. 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.

Descend the shot to the reef top at about 20 m, locate the lip and drop to the terrace at 25–30 m where the split in the wall forms the swim-through. Pass through it once with a torch, then work the overhangs either side at 25 m before coming back up onto the reef top. Launch a DSMB at 20 m and drift for the safety stop.

Dive Victoria Group (Portsea and Queenscliff) – double boat dives to the Heads walls most days of the week, site chosen on the morning by slack-water time and swell; Bayplay Adventure Tours (Portsea) – weekend and holiday charters; Red Boats (Queenscliff) – weekend Heads charters; Melbourne clubs (VSAG, Power House Underwater Club, Scuba Diving Melbourne) run their own boats here regularly.