Lonsdale Arches is the showpiece of the outer Lonsdale reef. About a kilometre from Point Lonsdale light on the Bass Strait side of the Heads, the sandstone reef has been 'carved into intricate structures including arches and tunnels', in the words of the Scuba Doctor listing, which calls the site spectacular and notes 'a couple of amazing swim-throughs that are very safe and easy to enter'. The depth is a comfortable 12–18 m, and the whole reef can be covered in a 40-minute dive.

The arches are natural bridges where the current has bored through ridges of rock, big enough to swim through in a relaxed horizontal position with light visible all the way; the tunnels are longer but still open at both ends and free of silt. What lifts them above ordinary swim-throughs is the growth: the strong tidal flow through the entrance creates 'a nutrient-rich environment and subsequently prolific marine growth', and the undersides of the arches are solid with yellow zoanthids, jewel anemones, orange sponges and bryozoans. Southern blue devils, magpie perch, silver sweep and longsnout boarfish are the regular fish; southern rock lobster, giant Australian cuttlefish, draughtboard sharks and Port Jackson sharks are common, and Australian fur seals from the nearby haul-outs often follow divers through the arches.

It is dived on the ebb – the outgoing tide leaves this side of the reef in relatively slack water – and the limit is swell rather than current, since the site is fully exposed to Bass Strait. On a calm day it is an easy, memorable Open Water dive and one of the most photographed sites in Victoria; Twin Bommies, Lonsdale Bommies and Castle Rock lie on the same reef and are commonly combined with it on an 'outside' charter day.

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 20 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 20 minutes to the site.

The site is outside the Heads about 1 km from Point Lonsdale light; charters run out through the entrance on the ebb and are 15–20 minutes from Queenscliff, 20–25 from Portsea. 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. An open-coast site: unworkable in a south-westerly swell above about 1.5 m or when the entrance is breaking; light northerlies give flat water and 15–20 m visibility.

Shot or anchor on the reef top at 12 m on the ebb slack. Navigate the reef by its structure: follow the main gutter to the first arch, pass through, continue to the tunnel and the second swim-through, then loop back over the top of the reef checking the ledges for crayfish and cuttlefish. Safety stop on the reef top at 5–6 m above the sand or under a DSMB.

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.