Thank you so much for replying - very much appreciated!
I first thought that he was in the Australian Light Horse, but when I contacted them, they said that his uniform was that of the South African Light Horse. The hand writing on the back of the photo states "taken before embarking on the Boer War", but rather confusingly, the photographer is listed on the back of the photo as being by "Rupert Kay, The Strand, Sydney". George Charles Behrens was born in Australia in 1875 and moved at some point to South Africa where he married a South African, and had two children.
I have also recently come across this medal ribbon - which I think is the The Queen's South Africa Medal? (I attach a photo).