Meet the site
Marmion Marine Park, declared in 1987 as Western Australia's first marine park, stretches about 20 km from Trigg Island to Burns Beach and protects more than 136 fish species, ten seagrass species and 21 seabird species. The park's reefs run in lines parallel to the coast, and Three Mile Reef is the outermost, roughly 3 nautical miles offshore. DBCA's park guide describes it, with Little Island, as an offshore reef with better visibility than the inshore sites.
The reef is limestone with ledges, overhangs, caves and swim-throughs, covered in kelp and red algae on top and sponges and sea fans in the shade. Divers report the species typical of Perth's offshore reefs: western blue devils, blue groper, buffalo bream, dhufish, pink snapper, samson fish, bullseyes, morwong and old wives, with western rock lobster and wobbegongs under the ledges and Port Jackson sharks in winter. Australian sea lions sometimes visit.
Because the reef lies well offshore, visibility is typically better than on the inshore reefs, but the site is exposed to swell and needs a calm morning. Perth operators such as Image Dive & Charters include Marmion's offshore reefs in weekend morning charters from Hillarys, returning before the sea breeze.
Three Mile Reef suits Open Water divers comfortable with boat diving, and it offers a good introduction to Perth's offshore limestone reefs without the longer run to Rottnest.
Published site-level detail, including depths and exact positions, is limited. The information here combines DBCA's description with general knowledge of the park's offshore reefs, so check depths on the sounder and ask the operator for the current dive plan.
Research this site before booking Stored site notes may be historical. These questions highlight gaps without turning old instructions into a current dive plan.
Three Mile Reef (Marmion) is recorded as Reef in Perth. The listed access is boat. Confirm the actual meeting point, route and permission to visit; a map pin does not establish any of these.
Topics flagged by the stored research
These are questions prompted by the existing notes, not a verified or complete hazard assessment.
- Other water users
- Ask about vessel activity, exclusion areas and the agreed surface arrangements.
- Lines and obstructions
- Discuss any known entanglement concerns and the suitability of the planned area.
- Planned depth
- Confirm the intended route and training requirements; the recorded maximum is not a target.
- Water movement
- Ask how swell and local exposure affect the proposed entry, exit and route.
Historical access notes exist for this record, but have not been independently verified for current use. Request current directions from the operator or site manager rather than following an old entry description.
Questions, answered
Is Three Mile Reef (Marmion) suitable for my certification?
The record lists Open Water. That does not establish suitability for every route or day. Share your training, recent experience and equipment needs with the operator and ask about the specific proposed dive.
Can I book Three Mile Reef (Marmion) through YouDive?
YouDive is an independent research directory. Contact an operator directly to confirm whether it currently runs the trip, the inclusions and the booking terms.
What you might see
Western blue devil, blue groper, buffalo bream, dhufish, pink snapper, samson fish, bullseyes, morwong, old wives, western rock lobster, wobbegongs, Port Jackson sharks, sponges, sea fans
Location
Offshore limestone reef line about 3 nautical miles off the Marmion–Hillarys coast, on the western edge of Marmion Marine Park, with ledges and caves and better visibility than the inshore reefs.
Facilities nearby
Hillarys Boat Harbour: ramps, parking, toilets, cafes, dive shops and air fills.
Before you make plans
Conditions, access and operator schedules can change. This is a location guide, not a dive plan. Ask a local operator about current conditions and suitability for your training and experience.
Sources & further reading
trailswa.com.auimagedive.com.auavenueperth.comRecord updated 26 Sept 2026. Source claims are being reviewed.
