Can you spot the now disused strip? I had a hunch but...
Postscript... I have received an email from reader Rob who has taken the trouble to sketch the runway, the test track and more (thanks Rob!).
In the image below the red line is the HF proving ground. Rob reports that it has leveled areas raised in places to keep it out of what were regular floods, presumably. The floods I remember well from my childhood. I'm sure it still happens but it used to be quite catastrophic and regular in the past, until remediation and flood management work was done.
The blue line is the actual airstrip. Again Rob reports that the surface is still quite visible in parts as you drive along Airstrip Rd. The kink at the southern end seems there for no reason, although it presumably made sense to someone when the land was parceled up and sold off over time.
Here's Rob's sketch:
Well worth a visit, I suspect!
Indeed why not visit the full list of Sydney's airfields and airports?