The class K^A was an improved successor to the class K with the same driver and cylinder size. Its main improvements were changed frames due to the cracks in the K, roller bearings on all axles, hydrostatic lubricators and an ACFI feedwater heater. It had a shrouding around the smokebox and on top of the boiler to hide the feedwater heater. 

33 locomotives of the class K^A were built between 1939 and 1945 at the NZR's Hutt Workshops on the North Island with parts from the Hillside Workshops on the South Island. Two more were built in 1950 which got Baker instead of Walschearts valve gear. Six more had been built at Hillside in 1939 which had got a trailing truck booster and thermic syphons, these were designated class K^B.

The K^A was only used on the North Island and the K^B on the South Island. Like the K, they were real mixed traffic locomotives suitable for freight and express working. All but the last few were built with coal firing and between 1947 and 1953, all remaining were rebuilt to oil firing. In the same process, the shrouding was removed.

As early as in 1955, the first services of these locomotives were replaced by D^A diesel locomotives. The K^A was withdrawn between 1965 and 1967, followed by the K^B between 1967 and 1969. Preserved are only the K^A locomotives with the numbers 935, 942 and 945, all of which have run in preservation.