It is a sewer vent... it was built in the 1920s. The sewer line for the new city of Canberra led from the provisional Parliament House, through Yarralumla, to Weston Creek. Hardships were encountered during the construction of the system when limestone caves were encountered along the way.
A short distance away, was the camp to house construction workers working on the sewerage tunnel. Westlake (pop. 700) was located in Stirling Park, today's Yarralumla. Westlake is a story on it's own...
There is a great document about the whole system in the ACT by engineers.org located here
Found on the Weston Creek side of the intersection of Cotter Road and Drakeford Drive.
(Opposite ACT RSPCA (here)
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