Reids Folly is a rock pinnacle on the northern side of Nepean Island. Oceanatik describes a pinnacle dropping to no more than 24 m and a large coral-covered archway partway down, and the Marine Life Network describes 'unique rock topography with archway' at 24 m. It is rated suitable for beginners in calm conditions.

The arch is the feature that draws divers. Coral-covered arches are a hallmark of Norfolk's diving – Cooks Arch, Turtle Arch and Little Organ all have them – and at Reids Folly the arch sits on the flank of the pinnacle, so the dive combines a swim-through with a circuit of the rock. The pinnacle's faces are covered with the mix of hard and soft corals, sponges and sea fans that Norfolk's reefs support.

Marine life is described simply as abundant, but the fish fauna of the Nepean Island sites is well recorded by Nigel Marsh at nearby Fish Bowl and The Crack: Galapagos sharks, kingfish and trevally, goldribbon cod, black cod, morays, striped boarfish, green turtles and a range of subtropical reef fish. The Marine Life Network lists a broader Norfolk species set including butterflyfish, wrasse, parrotfish, anemonefish, sweep, groper, morwong, rock cod and emperors.

The name suggests a local story, but its origin is not documented in the sources found. The site suits Open Water divers comfortable to about 20 m, with the deeper base better for Advanced divers. Like the other Nepean dives, it is reached from Kingston Pier in a few minutes and is dived when the southerly swell is down. Visibility is typically 15–45 m.

Norfolk Island is about 1,450 km east of Brisbane; flights run from Sydney and Brisbane (about 2.5 h; check the current carrier). A hire car is essential. Norfolk has no harbour: Norfolk Island Diving's open boat is lowered by crane from Kingston Pier (south coast) or Cascade Pier (north-east coast), whichever is in the lee of the day's wind, and divers board from the pier steps once it is in the water.

For the Nepean Island sites the boat launches at Kingston Pier and runs about 1 km south to Nepean Island – a trip of 5–10 minutes. Nepean is a small, uninhabited island within Norfolk Island National Park; landing is not part of the dive.

Norfolk's south coast, including Kingston, Nepean Island and Phillip Island, faces the prevailing south-east trade winds and southerly swell, which are strongest from May to September. A south-easterly over about 15 knots or a southerly swell above 1.5–2 m makes the boat launch at Kingston Pier unworkable and the southern sites rough; they are dived on northerly or light-wind days, often in summer. Surge is felt in caves and gutters with any swell.

Visibility is typically 20–40 m and can exceed 40 m in winter. There are no stingers, and cyclones rarely reach this far south, though ex-tropical lows can bring big seas in summer and autumn.

Descend onto the pinnacle and spiral down to the archway, swimming through it if conditions are calm. Explore the base to about 20–24 m, then circle back up the pinnacle and finish on its upper section for the safety stop.

Norfolk Island Diving (local instructor James and divemaster Mitch Graham) – small open boat craned in at Kingston or Cascade Pier depending on wind; runs part-time, mostly weekends, so book ahead by email (norfolkislanddiving@gmail.com).