Five Fathom Reef

Five Fathom Reef Dive Site Guide | Rockingham, WA

By ScubaDownUnder Team · 2026-04-14

[Five Fathom Reef](https://www.scubadownunder.com/dive-sites/five-fathom-reef) sits within the Shoalwater Islands Marine Park south of Rockingham, in the arc of protected water that spans from the mainland out to Penguin Island and Seal Island. The reef itself is a submerged platform that rises to around 6 metres from an 18-metre surrounding depth, and its position within the marine park means the biological community on its surfaces has been developing under formal protection for decades. For Perth divers it is among the most logistically accessible southern WA reef dives, a 45-minute drive south to the Rockingham boat ramp and a short boat run delivers a temperate reef in good condition with reliable sea lion encounters and a resident weedy sea dragon population.

The Shoalwater Islands Marine Park was established in 1990 to protect the ecological values of an area that includes significant seabird and marine mammal populations alongside the reef and seagrass ecosystem of the inner coast. The nearby colonies on Seal Island and Penguin Island contribute their animals to the water around Five Fathom Reef regularly. Australian sea lions from the Seal Island colony are a reliable presence, and little penguins are occasionally encountered in the water, swimming with an agility in their element that their waddling ashore entirely fails to suggest. The combination of seabird and marine mammal life in the surface and water column gives Shoalwater Bay a wildlife-density character that few near-Perth dive areas can match.

Weedy sea dragons (*Phyllopteryx taeniolatus*) are resident on the reef in the kelp and seagrass zones, and their presence in the temperate southern WA waters connects this site to the sea dragon community that runs along the continent's southern coast from SA across to WA. The Rockingham population is well established and reliably found by divers who slow down through the seagrass margins of the reef. The dragons drift with their algae-mimicking appendages held loose in the water, and a patient hover at the kelp edge typically reveals one to three animals on a productive dive.

The reef structure itself is a low-relief limestone platform with kelp coverage on the upper surfaces, sponge gardens in the more sheltered pockets, and encrusting reef community across the vertical faces. Wobbegong sharks rest on the reef structure in the characteristic flat, immobile posture of an ambush predator that has nowhere in particular to be. Port Jackson sharks aggregate in the gutters during the cooler months. The nudibranchs on the encrusting reef surfaces reward a patient search, Shoalwater is one of the better near-Perth sites for nudibranch diversity, particularly through the May–September window when the cooler water supports a wider range of species.

The fish life on the reef is the supporting cast, schools of zebra fish, breaksea cod, scalyfin and the southern wrasse species working the kelp margins. King George whiting and snapper are present in the deeper sand pockets surrounding the platform, and the seasonal pelagic schools, yellowtail, samsonfish, kingfish, pass through on the warmer-water periods. The contrast between the temperate reef community on the structure and the warmer-water visitors gives Five Fathom an interesting biological seasonality that varies the dive across the year.

Water temperature ranges from 16°C in winter to 22°C in summer, with a 5mm wetsuit adequate for the warmer months and 7mm preferred from June through September. Visibility is the variable that determines the dive quality, on a settled day in summer it runs 12–18 metres on the reef, with the better days clearing to 20+ metres. Strong wind and swell drop the visibility quickly in this comparatively shallow inshore setting, and the same easterly winds that flatten the open coast can stir the reef water on Shoalwater Bay's particular orientation.

For Perth divers building a southern-WA temperate-reef itinerary, Five Fathom is the close-to-home option, Penguin Island, Seal Island, and the Rockingham reef circuit deliver a marine park experience inside an hour of the city, and the combination of sea lions, sea dragons, and a healthy temperate reef community makes it a reliable training and recreational site that does not depend on a long drive south.

## Site Access and Logistics

Rockingham is approximately 45km south of Perth via the Kwinana Freeway. The reef is reachable by a short boat trip from the Rockingham boat ramp at Palm Beach or from the Mangles Bay marina; most dives are run from charter vessels rather than from private boats, given the reef location and current conditions. Open Water certification is appropriate for the depth.

Local operators: Several Rockingham-based dive operators run scheduled trips to the Shoalwater Marine Park reefs, and the same operators run the sea-lion and penguin snorkelling that uses the same boat ramps and waters. Combining a guided reef dive with a Penguin Island snorkel produces a full day of marine park experience for visiting divers.

A 5mm wetsuit is adequate from December through April; 7mm or a hooded vest is sensible for the cooler half of the year. The boat run is short enough that a full kit-up at the ramp is practical.

## Sources

- Department of Biodiversity, Conservation and Attractions WA, Shoalwater Islands Marine Park - Atlas of Living Australia, Weedy sea dragon (*Phyllopteryx taeniolatus*) and Australian sea lion (*Neophoca cinerea*) records - Parks and Wildlife Service WA, Penguin Island and Seal Island management plans - Marine Life Society of WA, Rockingham and Shoalwater site notes - Bureau of Meteorology, WA south-west coastal waters forecasts