This structure was part of the rebuilding of Millers Point after much of the suburb was razed as a response to the bubonic plague just prior to WW1. The plague was used as an excuse by the state government of the day to take control of the area and develop the maritime infrastructure that is so essential for an island continent. Hickson Road was carved through the suburb and ended at the edge of the harbour beneath the Dawes Point Battery which stood on the high part of Dawes Point facing to the east and was an essential part of the defence of the colony.
This entire wharf area was restricted, with the hoi-poloi kept out
From 1910 to 2005 the general public was kept out of Walsh Bay for their own safety and the security of goods being discharged.When the old Sydney Harbour Trust took charge in about 1913, they sliced through the area and carved out Hickson Road which, effectively, separated the residential area of Millers Point from the maritime area of Walsh Bay.
As I said in my post on Sydney Eye
Hickson Road - named after Robert Hickson, an Irishman who was the first president of the Sydney Harbour Trust - was carved from the steep slope that ran down to Walsh Bay. Henry Walsh - after whom the wharf area was named and who migrated to Australia from Ireland in 1877 - was the Chief Publc Works Engineer between 1901 and 1919 instigating improvements to the ports of Sydney and Newcastle.Now look at these diagrams:
See in the RHS photo how Hickson Road snakes through and does a big curve to the right at Pier 1. This is the location of the towerette. It is in line with the curve of the sandstone escarpment. Now look at the sandstone blocks embedded in the bitumen: they run from the towerette in the direction of the escarpment which has been "lined" with a very similar sandstone block. The LHS image shows just how intrusive Hickson Road was: how much land was resumed. The sandstone footings noticed by Sally - and totally overlooked by muggins here - are in line from the tower to the sandstone reinforced escarpment.
Looking at these three historic photographs:
There is a timber structure in three (small) sections that is very close to where the towerette now stands. This timber structure (to my eyes) is very similar to a structure on the Opera House walk which now sells coffee but whose original purpose was a tram control point going toward Fort Macquarie. These photos were taken as they were excavating for the southern abutment, the first in April and the other two in May of 1925. The more fascinating photo is the one in the middle which faces west. Look immediately beyond the diggings, yet before Pier 1: there is a small building with pointed eaves. This is what reminds me of a tram control point. Check them out in that old photo that you published of Railway Sq. Sally. Now look immediately in FRONT of this TCP ... dah-dah!! Note that the SHB did not disturb the railings.
I put these things together: people were supposed to be kept out; and, the escarpment ended in an entry point which was overseen by this towerette and (possibly) another towerette on the landward side - now lost to history, like so much else. My best guess: this was constructed immediately prior to WW1 and was part of a control gate, either to control trams or to keep people away from the wharf area, in which case I would expect a similar structure on the Barangaroo side. Now to find out where the trams ran in Millers Point. I think it is now time to contact someone in the council or the SHFA to see if they have anything to help us.