Catch Bag Reef takes its name from a lost bag. According to the Scuba Doctor listing a well-known cray diver, transferring gear to the boat, dropped his catch bag – containing an exceptionally large crayfish – over the side on this reef, and the name stuck. The site is a flood-tide drift over kelp forest in the Lonsdale Bight between Point Lonsdale and Queenscliff, at 12–24 m, and it also works as a slack-water reef dive.

The bottom is 'large kelp fronds with broken sandstone reef' – ridges and slabs of limestone with Ecklonia and other kelps on top and ledges beneath, running east from the shallow end of Lonsdale Wall, which is only 200 m away. The drift covers a lot of it: at a knot of flood a group passes over several hundred metres of reef in a 40-minute dive, and the kelp opens and closes to show southern rock lobster and abalone in the ledges, leatherjackets and wrasse in the fronds, and, in summer, 'large schools of yellowtail kingfish', which the listing singles out and which follow the flood in through the Heads. Where the kelp thins the sponge gardens of the bight take over.

It is a less-known drift than Boarfish Reef Drift and is used by clubs and by charters looking for a change of ground, or as a second dive after the wall on the same slack-into-flood sequence: dive the wall on the flood slack, surface, and drift Catch Bag on the young flood. The kelp makes the safety stop interesting – it is possible to hang in the canopy at 5 m – but it also means keeping the float line clear of the fronds. Within the marine national park, the crayfish and abalone are now protected.

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 12 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 12 minutes to the site.

The site is in Lonsdale Bight next to the shallow end of Lonsdale Wall, 12 minutes from Queenscliff. 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. Run as a group drift on a flagged float line; can also be done as a slack-water reef dive.

It is normally run as a flood-tide drift, with the incoming tide carrying divers along the reef towards Queenscliff. 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.

Inside the Heads it is sheltered from most swell, but a south-west swell over about 2 m still surges across the top of the wall and a westerly or south-westerly wind over 15–20 knots makes the boat work unpleasant; big seas outside drop visibility to a few metres for a day or two. 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.

The flood sets from the Heads toward Queenscliff across this reef, so the drift starts near the wall and finishes toward the bluff; also diveable at slack.

Enter up-tide near the Lonsdale Wall end on the early flood, descend the float line to 12–15 m and drift east over the kelp and broken sandstone toward Queenscliff, keeping the line as the group reference and watching the ledges for crayfish and abalone and the mid-water for kingfish in summer. Boat retrieves from the float.

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.