Up in the towers, a team of volunteer bellringers ring 14 antique bells that peal across the city in the key of C# major.
Below, in the vestry, sacristans lovingly prepare the books and vestments needed for Mass.
"It takes many, many hands to keep St Mary's Cathedral running smoothly," the Dean of the Cathedral, Fr Don Richardson, said.
St Mary's has been there almost as long as the city, so Sydneysiders tend to take the imposing, beautiful cathedral for granted.
But inside the cold sandstone, there's a bustling community bringing life to one of the city's most outstanding buildings.
Next year, St Mary's will celebrate its sesquicentenary.
It will be 150 years since the foundation stone was laid on land grudgingly given to the Catholic lower classes (the Anglican cathedral was built in what was then a better location, at Town Hall).