Cooks Arch is one of Norfolk's signature boat dives. Oceanatik describes 'a beautiful rock formation raising to 12 meters above surface and drops down to 21 meters below the surface', a pinnacle that intersects a reef system with multiple crevices and nearby caves, and an arch divers can swim through while looking at the surrounding coral and marine life. The Marine Life Network places it in Duncombe Bay, with rock walls from 12 to 21 m and abundant marine life.

The name links it to Captain James Cook, who sighted and landed on Norfolk Island in October 1774 during his second voyage; the Captain Cook Monument stands on the cliffs above Duncombe Bay.

The arch is the highlight. Swim-through arches are a feature of the north coast – Turtle Arch, Little Organ and Policemans Helmet are nearby – and at Cooks Arch the structure is large enough for a relaxed passage, with light coming in from both sides. The rock walls and crevices are covered in corals, sponges and fans, and Oceanatik describes an excellent display of marine life. Norfolk's reef fauna includes drummer, sweep, wrasse, butterflyfish, groper, morwong, rock cod and moray eels, and kingfish and trevally hang around the pinnacle.

Oceanatik rates the dive suitable for beginners, and the maximum depth of 21 m keeps it within Open Water range. The north coast is sheltered from the prevailing south-east trades, but northerly swell creates surge in the arch and caves, which should then be avoided. The boat usually launches from Cascade Pier. Coordinates are approximate for Duncombe Bay.

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 rock and swim through the arch in calm conditions, then circle the pinnacle along the reef at 12–21 m, looking into the crevices and nearby caves. Finish on the upper rock 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).