James Aldridge (Harold Edward James Aldridge, b. 1918) was born in Australia, though now lives in London.  He was a journalist and war correspondent before he started writing full time.  His Quayle family sequence is set in a small town on the Murray River, and his books tend to reflect the Australia in which he grew up.  Stephanie Nettell said:

“Horses are a recurrent Aldridge motif.  The Broken Saddle is very much the story of his own relationship with one special pony as a boy, with the same fierce feeling for the wild country of Australia as in The True Story of Spit McPhee.”

Probably his best known book is Ride a Wild Pony (originally published as A Sporting Proposition).  It was made into a film in 1975 with the title Ride a Wild Pony, and Penguin published it under this title in paperback.  

Sources:

Contemporary Children’s Authors, ed. Chevalier, 1989 3rd edn.