This image is from State Records NSW ( Digital ID: 4346_a020_a020000128.jpg.)
You can check it here at State Records NSW's photo investigator archive or here at their Flickr photos. Used here with permission.
The photograph is undated.
Here is my January 2009 view:
Now, let's see how far we get trying to date the first photograph.
At State Records NSW, the photograph comes from Series No: 4346, images which date between 1880 and 1940.
The obvious difference between the two photographs is the addition of the clock tower.
Now, the building is probably listed under the Heritage Act, NSW.
"... and 1889 when construction of the Post Office was finished. The office was ready for occupation on 31 January 1889, with the cost of the Post Office section being £4500. A clock was installed over the front entrance to the Office, later being transferred into a new tower. The clock tower was built over the entrance in 1895 at a cost of £965, with the clock being installed in July 1900."
Therefore, the photograph was taken between 31 January 1889 and 1895 inclusive.