Just to add,
If your David Brown turns out to be David Browning, then there's a great deal of family information on those NAA records, including an extract of the marriage certificate in England in 1919,- that gives his wife's nee name, and the names and occupation of both their fathers. And of course on his 1914 enlistment there's his mother who was nominated as his n o k. It shows he was born at Blayney in central western NSW. Perhaps this would be the
1891 birth registration at Carcoar (near Blayney) to Richard and Elizabeth of a son named David Browning (9967/1891 at NSW BDM reference)....
His enlistment definitely shows 2nd Light Horse Regiment, and you have mentioned the possibility that he used an alias when enlisting (Brown v Browning). The naa papers also give his age (23 years 4 months in Aug 1914) and his height 5 ft 8 1/2 inches. He weighed 164 lbs, had fair complexion, brown eyes and fair hair. And he was C of E. I cannot tell from the photo if any of that matches up with your chap.
I stress, there's NO way of knowing if your David Brown was connected to this David Browning, just on what I have posted,, SORRY but I don't know of any more ways I can try to help on this thread.
Cheers, JM