Wild kelp forests and predator free islands off the Coromandel coast
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Overview
The Mercury Islands, or Nga motu o Ahuahu, are a cluster of predator free islands sitting off New Zealands Coromandel Peninsula. Based out of Whitianga in Mercury Bay, divers and snorkellers cross 15 km of open water to reach sheer walls, kelp forests and fish filled pinnacles that feel far wilder than the mainland coast. Great Mercury Island (Ahuahu) welcomes low impact day visitors, while the six smaller islands are DOC managed nature reserves with strict no landing rules and globally significant seabird and reptile communities. Underwater, the Mercs mix easy sheltered bays with advanced, current swept sites that attract schooling kingfish, trevally, rays and the occasional orca. Exotic Caulerpa restrictions and a no take zone around Ahuahu make this a true look dont touch destination for conservation minded divers, freedivers and snorkellers.
The Mercury Islands (Mercs) are a small archipelago 6 km to 10 km off the eastern side of the Coromandel Peninsula, best accessed from the harbour town of Whitianga in Mercury Bay. The group includes Ahuahu/Great Mercury Island plus six smaller DOC managed nature reserves (Red Mercury, Korapuki, Green, Middle, Stanley and Double Islands). All except Ahuahu are strictly no landing to protect fragile seabird and reptile populations.
Ahuahu itself is privately owned but allows carefully managed day visiting by boat, with strong biosecurity requirements to keep the island pest free after successful eradication of rats and feral cats in 2014. The islands sit within the Hauraki Gulf Marine Park and feel much more remote than their distance from the mainland suggests.
The Mercs are classic temperate New Zealand diving: volcanic and sedimentary reef, vertical walls, boulder fields and kelp forests dropping into sand at 15 m to 40 m. Coralie Bay and Peach Grove offer sheltered bays with extensive Ecklonia kelp and sand channels, while advanced sites like The Book Case and Never Fail Rock are steep walls and exposed pinnacles swept by current and swell. Expect schools of blue maomao, trevally, kahawai, pink and blue maomao, butterfish, demoiselles, snapper, crayfish and occasional pelagics.
Visibility varies from 8 m on a green day to 20 m plus in stable summer conditions. Water temperature ranges from around 15°C in late winter to 21°C in late summer.
Six of the seven Mercury Islands are DOC nature reserves with no public landing and strict pest free rules, while Ahuahu has its own biosecurity protocols for visiting boats and day trippers. In addition, a Controlled Area Notice for exotic Caulerpa around Great Mercury Island creates a no take zone where removing any marine life (fish, shellfish, crayfish, seaweed) is illegal and anchors, chains and gear must be thoroughly cleaned before leaving.
These layers of protection mean the Mercs feel noticeably more fishy and intact than many inshore New Zealand reefs. They also demand high standards of diver behaviour: perfect buoyancy, no collecting and meticulous decontamination of dive, snorkel and boating gear.
Although the destination focus is the Mercs, most visitors base themselves in Whitianga or Hahei and combine offshore days with shore based adventures. Te Whanganui a Hei / Cathedral Cove Marine Reserve offers easy snorkelling and iconic coastal scenery; Hot Water Beach delivers geothermal hot pools in the intertidal zone; and Coromandel Forest Park adds waterfalls and kauri forest hikes. This mix of offshore wilderness and accessible topside highlights makes the Mercury Islands area an unusually well rounded trip for mixed dive and non dive groups.
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Last updated: November 21, 2025 • 8 sources
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Trip callouts
Pest-free offshore islands keep bird and lizard colonies intact and the surrounding reefs lively.
From Whitianga, day boats reach bays, walls, and pinnacles in about an hour, delivering offshore viz and schooling pelagics.
Great Mercury’s Controlled Area bans take and requires strict gear cleaning, keeping it a rich no-take zone.
Kelp, sponges, and boulder walls give temperate macro habitat and playful reef fish instead of tropical coral.
Whitianga supplies dive shops, ramps, beds, and restaurants, plus Mercury Bay Museum for mixed groups.
scuba
Why Mercury Islands for Scuba Diving
For certified divers the Mercs pack a lot into a compact area: sheltered bays ideal for training, caverns and arches, walls plunging past 30 m and offshore pinnacles that draw kingfish, trevally and big schools of blue maomao. Dive Zone Whitianga and other local charter operators run day boats that choose sites based on swell and wind, so you can usually find a lee even when the forecast looks marginal. This is classic New Zealand temperate diving with a strong conservation story layered on top.
freedive
Why Mercury Islands for Freediving
For experienced freedivers the Mercs offer steep topography, relatively clear summer water and plenty of fish life, all within a short drop from the boat. Walls around The Cathedral and Ahuahus eastern bays give easy line placement with depth in the 20 m to 35 m band, while sheltered coves provide relaxed reef cruising for photography and spearfishing outside the Great Mercury Caulerpa zone. Cold water, current and boat traffic mean this is best for solid intermediate and advanced freedivers training with a buddy or local instructor rather than first timers.
snorkel
Why Mercury Islands for Snorkelling
Snorkelling the Mercs puts you eye to eye with kelp forests, schooling reef fish and rocky coastlines that feel wild but are still reachable on day boats. In the right bay, you can fin over shallow reefs at 2 m to 6 m while watching snapper, blue maomao and butterfish glide through the kelp, or cruise sandy coves on Ahuahu with seabirds wheeling overhead. Nearby, the Te Whanganui a Hei / Cathedral Cove Marine Reserve adds shore based snorkel trails at Gemstone Bay, offering an easy backup for windy days offshore.
topside
What to do when youre not in the water
The Mercs sit at the heart of one of New Zealands classic summer playgrounds. Back on shore, Hot Water Beach, Cathedral Cove, Mercury Bay Museum, local vineyards and Coromandel Forest Park hikes all sit within an easy drive of Whitianga. Ahuahu itself is on a path toward International Dark Sky Sanctuary status, so overnight yacht charters can add serious stargazing to your logbook.
