In the early nineteen-twenties, five were given superheated boilers. Starting in 1929 some of the class were reboilered with wide fireboxes. The first to be upgraded was B 306, re-entering service in March 1930. The overhaul involved the installation of wider fireboxes and superheated boilers, increasing the boiler pressure to and generating a tractive effort of . B 304 was similarly overhauled in 1931 and B 307 followed in 1935, and then a considerable length of time elapsed until B 303 in 1948 became the fourth and last to be upgraded. As rebuilt they were rated for on the level and on the 2.2 percent (1 in 45) over the Reefton Saddle.