This reminds me of the Grand Archway at Jenolan (is that the name ?!) and a couple of similar large limestone features in England, but also of one that I visited recently in NSW.
I was passing near Canberra during a non-caving holiday and visited a place called "London Bridge" (yes, yet another one!). It is a natural limestone archway approx 10m wide and 10m high with a creek running through it. Quite impressive. There are other minor holes in the rocks that form the feature, but no significant nearby caves (as far as I could see).
This place is about 20km south of Queanbeyan, at the upper end of Googong Reservoir.