All ten L class locomotives were built by the Avonside Engine Company in Bristol. The first L class built, entered service in 1878 and another nine L class locomotives were ordered. In 1893-94, three of the L class 2-4-0T locomotives were rebuilt to a 4-4-0 wheel arrangement at Newmarket workshops, with larger boilers and enlarged cylinders. This new design was classified La, but their limited coal bunker capacity remained a drawback. The solution was to add a trailing pony truck to accommodate a larger coal bunker, converting them to a 4-4-2T wheel arrangement. A further four L class locomotives were similarly converted. Three were not rebuilt, but sold to the Public Works Department between 1901 and 1903. Three new locomotives were built in 1902-03. When the conversion program was completed in 1903, the classification for all ten remaining NZR locomotives was changed back to L.