Mushroom Reef is a small site between Mushroom Rock and Peak Island. Marinelife.org.au describes an 8 m reef with a narrow crevice cutting through the island, packed with marine life, which is good in suitable weather but too narrow and dangerous in swell.

The crevice concentrates fish and invertebrates in the way that narrow limestone channels do: pullers, scalyfin, wrasse and bream sheltering in the walls, with sponges and encrusting life in the shaded parts. It is a short, interesting add-on to the Toba wreck and Mushroom Rock, all within a few hundred metres of each other.

The risk is surge. Even modest swell accelerates water through narrow gaps, and a diver caught in the crevice can be pushed against the rock. Treat it as a flat-calm-only site and do not enter the crevice unless conditions are benign.

Published detail is limited, so the position and depths are indicative.

Hamelin Bay was a timber port from the 1880s, when a jetty built in 1882 and extended in 1898 served M. C. Davies' milling operations. It was notorious for wrecks in bad weather because it is exposed to the north-west gales of winter storms; the 22 July 1900 storm alone wrecked several ships, and only the old jetty piles remain today. The bay is now a recreation zone in the Ngari Capes Marine Park, and every wreck in it older than 75 years is protected. Combine it with Mushroom Rock and the Toba on the same outing, all within a few hundred metres of the ramp.

Hamelin Bay is on the coast west of Karridale, about 35 km south of Margaret River and 30 km north of Augusta, reached via Caves Road and Hamelin Bay Road. Park at the boat ramp/jetty area beside the Hamelin Bay Holiday Park. Launch at the Hamelin Bay boat ramp - marinelife.org.au warns it is narrow, slippery and swell-exposed, and a 4WD may be needed.

Mushroom Reef lies between Mushroom Rock and Peak Island, a few hundred metres from the ramp; anchor on sand.

Good in suitable weather, but the crevice is too narrow and dangerous in swell (marinelife.org.au). Only in flat conditions. General pattern on the Capes and Leeuwin coast: summer mornings often have light easterly (offshore) winds before a south-westerly sea breeze, and long-period south-westerly groundswell can arrive on otherwise calm days; winter fronts bring big swell and north-westerly winds.

Check the Bureau of Meteorology swell forecast, not just the wind.

Descend on the reef, examine the entrance to the crevice and, only in flat conditions, move slowly through it; otherwise explore the reef around it. 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.

No operator; private boats. Nearest dive services: Swan Dive (Busselton/Port Geographe) for fills, hire, nitrox and charters; no dive shop in Margaret River or Augusta.