Moon Ledge is Hervey Bay's best natural reef dive. The Fraser Coast diving guide describes it as a coffee-rock ledge stretching for some hundreds of metres, with depths from 10 m to more than 21 m, shallow caves and fallen boulders along its face. Marinelife.org.au adds that the coffee rock – compacted, iron-stained sand typical of the Great Sandy region – has eroded into spires and craters, giving the site more relief than anything else in the bay.

The ledge supports soft corals and feather-duster worms, and the caves and boulders shelter schools of large fish. Cod are a particular feature, along with snapper, angelfish and turtles. Because the ledge is long, a dive is a linear swim along the face and back, with the depth easily controlled by moving up or down the rock.

Visibility is the main variable. Marinelife.org.au warns it can be as low as 3 m after storms; on a calm neap tide in winter it will be better. Tidal current runs across the bay, so the site is dived around slack water.

Moon Ledge features on Hervey Bay Dive Centre's list of local dives from Urangan and makes a good contrast to the artificial structures of Roy Rufus – natural topography, bigger schools of fish and interesting geology. It suits Open Water divers with a guide, and more experienced divers who want to explore the deeper base of the ledge beyond 20 m. The coffee rock is soft and fragile, so keep off the face and avoid kicking up the fine sediment at the base.

Hervey Bay's boat dives leave from Urangan Boat Harbour (Great Sandy Straits Marina / Boat Club Marina, Buccaneer Drive, Urangan), at the eastern end of Hervey Bay's Esplanade about 10 minutes' drive from Pialba; there is free parking at the marina and a public boat ramp in the harbour. Hervey Bay Dive Centre runs its local wreck and reef trips from here. Moon Ledge is included in Hervey Bay Dive Centre's local-dive list; its position is not published, so dive it with the operator or obtain the mark locally.

Anchor in sand off the ledge, not on the rock.

Hervey Bay lies in the lee of K'gari, so ocean swell is rarely a problem; the limits are wind, tide and visibility. South-east trades over about 15-20 knots kick up a steep, short chop across the bay, and strong northerlies do the same in summer. Tides in the Great Sandy Strait run hard, so dives are timed for high-water slack – the Roy Rufus reef is dived only around high tide – and neap tides give the calmest water and best visibility.

Visibility is modest, typically 5 m and rarely above 10 m, and drops further after rain when the Mary River and creeks discharge into the bay (December-March). Winter (June-October) is generally calmer and clearer, and coincides with humpback whale season.

Descend to the base of the ledge and follow it with the rock on one shoulder, checking the shallow caves and boulders for cod and turtles. Turn on a third of your gas and return along the top of the ledge at 10-12 m. Ascend on the anchor line or an SMB.

Hervey Bay Dive Centre (Urangan Harbour) – 'Local Wreck Dives' trips to the Roy Rufus Artificial Reef and nearby sites (Goori, 3 Ships, K'gari, Iron Maria, Hardie and Simpson reefs, Snapper Grounds, Moon Ledge) on demand, timed to the high-tide slack; private boats also use the site.