Four Mile Reef is the main natural reef dive out of Busselton and a good counterpoint to the jetty. Swan Dive describes it as a 15-20 m site that is full of life, with large corals, schools of fish and tiny nudibranchs, and lists starfish, turtles and coral formations among the sightings. Octopus Garden Dive Charters in Bunbury also lists 'Busselton 4-mile reef' at 15-18 m. The name reflects its distance offshore in Geographe Bay.

The reef is part of the extensive system of limestone reefs and seagrass that makes Geographe Bay one of the richest temperate bays in Australia, influenced by the warm Leeuwin Current. Coral cover here is unusually good for a temperate site, and the sponges and ascidians attract a variety of nudibranchs, which makes it popular with macro photographers. Schooling fish such as bullseyes and buffalo bream, and western blue devils and wrasse, are common.

Swan Dive requires divers to be Open Water certified, to have dived in the past 12 months and to carry a DSMB and reel, reflecting the offshore setting and the risk of surfacing away from the boat. It suits Open Water divers who want a genuine reef dive near Busselton and is often combined with the HMAS Swan on a double-dive day.

Early starts are the norm because the sea breeze usually arrives by late morning.

Geographe Bay is unusual on the WA coast because it faces north, so the prevailing south-westerly swell and sea breeze that close the ocean coast are largely blocked by Cape Naturaliste. That shelter, plus the warm Leeuwin Current, is what makes the bay's reefs, jetties and wrecks diveable for much of the year, and why winter north-westerly storms are the main thing that shuts it down.

Swan Dive departs from the Busselton foreshore (Geographe Bay Road) for a short cruise through Geographe Bay past Busselton Jetty; the whole trip takes about 2.5 hours. Octopus Garden also visits from Bunbury. Private boats launch at Busselton, Port Geographe or Dunsborough ramps and locate the reef by GPS/sounder.

Boat ramps along Geographe Bay include Bunbury (Casuarina), Busselton, Port Geographe, Quindalup and Dunsborough; on the Capes coast the Canal Rocks, Gracetown and Hamelin Bay ramps are small and swell-exposed. Check the forecast, carry a marine radio and log a trip plan with the local Marine Rescue group.

Swan Dive recommends early-morning dives because of the site's location - the afternoon sea breeze builds chop across Geographe Bay. Sheltered from south-westerly swell but exposed to north-westerly winds and winter storms. General pattern in Geographe Bay: sheltered from south-westerly swell by Cape Naturaliste; summer mornings are calm before the afternoon sea breeze, while winter north-westerly fronts roll swell and chop into the bay and reduce visibility for several days.

Descend with the guide onto the reef at 15 m, work slowly across it looking for nudibranchs on the corals and sponges, and deploy a DSMB for the ascent as required by the operator. Agree a turn pressure before descending (half gas, or a third where current is likely), carry a DSMB, and make a 3-minute safety stop at 5 m.

Swan Dive (Busselton) - guided dives most weeks (OW+, dived within 12 months, DSMB and reel required); Octopus Garden Dive Charters from Bunbury occasionally.