Turtle Arch is a pinnacle near Cooks Arch on Norfolk's north coast, named for its turtle-like shape. Oceanatik describes a formation rising several metres above the surface and descending to about 21 m, covered in healthy marine vegetation and corals with abundant marine life, and rates it suitable for divers of all skill levels. The Marine Life Network says it is similar to Cooks Arch and lies to the east of it.

The north coast pinnacles share a character: volcanic rock rising sheer from the sea, with the underwater faces covered by algae, hard and soft corals, sponges and sea fans, and arches or swim-throughs worn through them. Turtle Arch's name suggests both its shape above water and, possibly, an arch below, although the sources do not describe a swim-through explicitly. Green turtles are regularly seen around Norfolk and may also contribute to the name.

Fish life follows the island pattern: drummer, sweep and wrasse in the shallows, groper, morwong and rock cod around the base, moray eels in the cracks, and kingfish and trevally circling the rock.

It is a straightforward dive for Open Water divers in calm conditions, with a maximum of 21 m. Like Cooks Arch, it is usually dived from a boat launched at Cascade Pier and is sheltered in the prevailing south-east trades, but northerly swell makes it surgy. It pairs naturally with Cooks Arch as a two-dive morning. The coordinates are approximate. Visibility along this stretch of coast is typically 15–45 m, so the whole pinnacle can often be seen from the surface.

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.

North-coast sites are normally reached from Cascade Pier (end of Cascade Road, about 10 minutes' drive from Burnt Pine): the boat runs north-west along the cliffs to the Duncombe Bay area below the Captain Cook Monument, roughly 15–25 minutes. The coast here is within or beside Norfolk Island National Park.

Norfolk's north coast, from Duncombe Bay west to Anson Bay, is in the lee of the prevailing south-east trade winds, so it is the most reliable part of the island for much of the year. It is exposed to northerly and north-westerly winds and swell, more common in summer, which break on the cliffs and pinnacles and cause strong surge. Anything over about 1.5 m of northerly swell closes the boat sites; the beaches need a flat day.

Visibility is typically 15–45 m and often best in winter. There is no stinger season.

Descend beside the pinnacle and circle it at 12–21 m, looking at the coral and marine vegetation; swim through any arch only in calm conditions. Finish on the upper face 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).