Is this a military airstrip from WWII? Or a Forestry Commission effort that's now been decommissioned? Or (last guess) some left-over infrastructure from the construction of Mangrove Creek Dam? Whatever the origin or original purpose, it's clearly a 2-runways crossing setup fairly deep inside MacPherson State Forest.
Unless you had a reason to hide in fairly rough terrain I can't see much point to it. Maybe it's my imagination!
Here is an updated list of Sydney's airports.
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