It's the official "street" language of Sydney – but only a select group of people know how to speak it. It could almost be described as underground if it wasn't so clearly, well, on the ground.

Walk towards Town Hall along Kent Street and you'll see the symbols everywhere. Cross Pitt Street near Circular Quay and you can't miss them. Those painted lines and numbers scrawled on the pavement might look like gibberish graffiti to the average pedestrian, but they are in fact a vital part of the city's construction network.

But what do they mean?

The location marks indicate what is underground, whether there are sewer pipes, telecommunication cables or gas pipes.