In 1894 Riley was appointed Bishop of Perth, then the largest Anglican diocese in the world, with an area of 1,000,000 square miles (2,600,000 km2) and a scattered population of about 100,000.

Riley arrived in Western Australia on 3 February 1895[1] and found that the diocese had few clergy, little money, and poor means for organizing religious services for the now rapidly increasing population - due largely to the gold rush.

[1] He was also noted for his close association with Sir John Winthrop Hackett in working for the establishment of the University of Western Australia.

Riley toured the UK, France & Egypt for 3½ months in late 1916 early 1917 enquiring into the administration of each theatre's Chaplain's Dept, returning to Australia in February 1917.

Riley's usually robust health began to fail and his impending retirement was announced shortly before his death on 23 June 1929.