Following World War II, North Island coal was in short supply and, as an experiment, K 927 was cohverted into an oil-burner in October 1945. Results were so successful that all K and Ka locomotives were converted, much to the delight of the firemen. Ka's 958 and 959 were the onls 4-8-4's built as oil-burners. In the South Island quality West Coast coal was in good supply, and the six Kb's remained coal burners. In 1953 when the North Island coal was again plentiful and oil prices escalated, K's 901, 915, and 925 were converted back to coal burning, but the engine crews refused to work them. They were converted back to oil in 1954 when oil prices fell.

A total of 71 class K, Ka, and Kb 4-8-4, three and a half foot gauge locomotives were built between 1932 and 1950, most by the Hutt Workshops on the North Island, but six were built at the Hillside Workshops in 1939 for use on the South Island. Retirements and scrapping began in 1964, as the GM G12 diesels began arriving. Five 4-8-4 locomotives were saved from scrapping; the original K number 900, Ka's 935, 942, 945, and Kb 968.