Since the Western Australian Government Railways didn't have enough operational locomotives after World War II, they ordered a modernized variant of the pre-war class P that was to be designated class Pm. Ten of 35 locomotives were to be built at their own Midland Railway Workshops. But due to insufficient capacities, all 35 were delivered by North British and put into service in 1950 after they had been shipped to Australia.

As with the P and Pr, 19 Pm were built with plain bearings on the driving axles and roller bearings on the carrying axles, while the 16 Pmr had roller bearings on all axles. One Pm was later rebuilt to a Pmr at the Midland Railway Workshops. Due to their rough running characteristics, they were quickly relegated to express freights and used on the Great Southern and South Western lines. Withdrawals came between 1970 and 1972.