After the class Q had established the Pacific wheel arrangement for express service in New Zealand, the NZR ordered four-cylinder compound express locomotives. Since the Vauclain compound system was not liked by the NZR, the new locomotive was designed as a De Glehn compound with its high pressure cylinders on the inside. In this configuration, this class was likely the first narrow gauge compound to have inside cylinders. As the tiny class A of 1873 had been fully withdrawn by this time, the newer class also got the letter A.
The first eight were built at the NZR's Addington Workshops and fifty more were built by A & G Price. One of these was built as a superheated simple two-cylinder, but this one did not show much better performances than the saturated four-cylinder compounds. The first 27 compounds had intercepting valves which allowed simple operation at all speeds. The last 30 did not have these valves and were originally designated class AD, what was later changed to A, too.
Originally, 50 were placed into service on the North Island and ten on the South Island. From 1931, 38 more came to the South Island. When the boilers of individual locomotives had worn out, they were replaced with superheated ones. Between 1941 and 1949, all were rebuilt to two-cylinder simples since the maintenance of the inside cylinders was too costly. Withdrawals on the North Island were completed in 1961 and in the South in 1969. A 423 and A 428 were preserved, of which A 428 is still operational, but under overhaul since 2023.