Like a speeding bullet, the fairy penguin darts through the turquoise shallows in pursuit of a shoal of fish, its distinctive blue markings shimmering in the sun. A few metres away, a pelican preens itself on a white-streaked rock, and a group of pied cormorants air their stubby wings. It may sound like a snapshot from a national park, but this is a pocket of Sydney harbour on a crisp winter's day.
After decades of being used as a dumping ground for industrial and household waste, the harbour - more correctly known as Port Jackson - has never been cleaner.
Storm-water barriers installed in the late 1990s prevent thousands of tonnes of effluent from being disgorged into the harbour and ocean every year. Longer sewage outfall pipes have resulted in a dramatic improvement in the quality of water along the city's beaches.
And the long-running drought which is gripping eastern Australia means that the water is crystal clear because there is less dirty water reaching the harbour after heavy rain.
Marine wildlife is 'voting with its fins' and returning to the harbour, according to the New South Wales environment department. In recent years migrating humpback and southern right whales have begun entering the harbour, seeking a rest on their long migration from Antarctica to Queensland.
Pods of dolphins are routinely seen at Bondi and Manly, Sydney's two most famous beaches. Surfers and kayakers have become accustomed to having dolphins frolic around them. Sharks appear to be increasing in number, too, including species such as hammerheads and bronze whalers, which are aggressive and dangerous to humans.
Experts say the presence of dolphins and sharks are indicators of the health of the marine environment, because they would only return to the harbour if there was plenty of fish around.
Another creature to make a spectacular comeback is the native water rat, which has webbed feet, orange belly fur and an otter-like appearance. It was spotted five years ago after an absence of more than 70 years, and has since been seen regularly around the harbour's many islands.
The harbour - which still hosts container ships and other commercial traffic on a daily basis - is home to no less than 600 fish species. Actually, make that 601. Scientists announced last week that they have discovered a brand new species of scorpion fish, just a few centimetres long and with a prickly spine.
They say it is astounding that such a discovery can still be made in the backyard of Australia's largest city. In tribute, they have named it insperatus - from the Latin for unexpected.