Lion Island is the distinctive sphinx-shaped island at the mouth of Broken Bay, visible from both the Central Coast and Sydney's northern beaches. NSW Marine Estate documents describe it as a nature reserve (originally a fauna reserve from 1956) surrounded by estuarine reef, and note that scuba diving and snorkelling are permitted within the 45-hectare zone extending 200 m from the island, while hand gathering and collecting would be prohibited under proposed special purpose zoning, and boat speed limited to 8 knots to protect the island's threatened little penguins. The island's rocky shores drop to estuarine reef, a habitat influenced both by the ocean and by the Hawkesbury River.

Kelp and algae cover the shallower rocks, and sponges, ascidians and encrusting growth increase with depth. Fish include those typical of Broken Bay and nearby Barrenjoey: luderick, bream, silver sweep, yellowtail, red morwong, blue groper, leatherjackets and, in the deeper water, snapper and mulloway. Wobbegongs and Port Jackson sharks rest on ledges.

Little penguins may be seen on the surface or underwater near the island, especially at dawn and dusk. The island also has cultural significance, with shell middens and spiritual sites valued by local Aboriginal communities. Detailed dive descriptions of Lion Island are scarce, and conditions in Broken Bay vary widely with tide and river flow.

Divers need their own boat, should choose the lee side, and should plan around the tide. For those who make the effort, it offers an uncrowded, scenic reef dive at the meeting of the Hawkesbury River and the Pacific, and a chance to see penguins.

Lion Island can only be reached by boat. Launch at Patonga, Ettalong or Woy Woy on the Central Coast side, or Palm Beach, Newport or Church Point on the Pittwater side; the island is about 2-3 km from Patonga and Palm Beach (10 minutes by boat). Anchor carefully on sand clear of the reef, observe the 8-knot speed limit proposed near the island to protect penguins, and do not land - the island is a nature reserve closed to the public.

Lion Island sits in the mouth of Broken Bay, where the Hawkesbury River meets the ocean. Swell from the south and east wraps into the bay and surges on the eastern side of the island; choose the lee side for the day's swell and stay away from breaking water. Visibility depends on the tide and rain: best on the flooding and high tide after dry weather when ocean water pushes in, and poor on the ebb or after heavy rain in the Hawkesbury catchment.

Tidal currents can be strong around the island's ends.

Choose the sheltered side for the day's conditions, anchor on sand, and follow the rocky reef around the island edge at 6-15 m, keeping an eye on the current around the ends. Return to the anchor before the tide turns.

Special purpose zone proposed under the Hawkesbury Shelf Marine Bioregion management - check current NSW Marine Estate rules before diving.

No regular dive operator; occasionally dived from private boats by Central Coast and northern Sydney divers.