Last December I joined Bill Harvey on a visit to Lennox Bridge west of Sydney.  Bill has now documented the visit in his Bridge of the Month site, including history, comments on the structural form, and a dynamic 3D model of the structure:

Lennox Bridge is an integral part of Mitchell’s Pass, the old road into the Blue Mountains (I presume through them as well, but I didn’t get that far). It is the oldest surviving stone bridge in mainland Australia (an older one off the mainland is in Richmond, Tasmania, and was opened eight years earlier in 1825

Visit Lennox Bridge for more details, photographs, and view the 3D model at https://skfb.ly/6RZEx .

One of my photos, with Bill constructing his 3D model: