In the late 1890s, New Zealand's national network was expanding at a great rate and demand for services on existing lines was rising.

However, the size of the locomotive fleet was inadequate to handle the demand - this was at least in part due to the economic difficulties created by the Long Depression.

The New Zealand Railways Department (NZR) had built the WA class in its own workshops, but desperate for more engines, went shopping overseas for more.

Although initially classified as WA, they were re-classified WB in 1900–01 to avoid any confusion with the earlier locomotives constructed in New Zealand.

On the isolated Westport section, the dominant traffic was coal from the various inland mines served by rail, and the WBs worked these services for a number of decades until they were displaced in the 1950s by the WW class.