There was a channel for water to flow out the north eastern corner (near the A marker)
I found a post maker with this diagram, seeming to show the route.
You can read more about Woorabinda Lake Reserve here
Interesting, that diagram could
suggest that the natural watercourse (the top end of Aldgate Creek, eventual destination the Onkaparinga at Mylor) was used between there and Aldgate station instead of an aqueduct, kind of like a small-scale version of the way the Onkaparinga is used that way as part of the link between the Murray and Happy Valley Reservoir for the mains supply to most of Adelaide. Good thing we switched to diesels, I can't imagine a solution like that working too well for steam locos these days with the greasy run-off you get now the hills are essentially part of the metropolitan area.
Alternatively, there could have been an aqueduct either following the railway or following the course of Aldgate Creek to Madurta, with that diagram just pointing out the overflow route for when the dam got too full. The outlet for that could well have been buried underneath the embankment forming the western side of the lake above the railway.
If I was in charge of building it today, I would prefer to run a closed pipeline alongside the railway (using the 2-2.2% grade to my advantage) from the nearest point to the lake, to keep it easily accessible for cleaning and maintenance by virtue of being within the land owned by the rail company for the whole distance from the dam to the station. I would also consider setting up a LNWR-style water trough at Aldgate for non-stopping trains as well as a traditional crane for those that stopped.
That weekendnotes page is rather interesting, mentioning the Mackereth Cottage and the Almanda Silver Mine at Scott Creek Recreation Park as being nearby to Woorabinda when they are 14 kilometres away by what can be a rather confusing road network! That's a lot further than many other more interesting spots which aren't mentioned, such as Mark Oliphant Recreation Park which is only 20 minutes' walk away.