The first member of the B class, nicknamed Snake, was introduced in September 1874 and it worked in Auckland; it was followed by its partner Lady Mordaunt on 5 April 1875, which was based in Otago.

The Auckland-based Snake became the sole member of the B class in 1876; Otago locomotives were omitted from the national classification initially, but in 1877, it was necessary to include them as the Main South Line linked the southern provinces.

Both of these methods of allocating classifications quickly proved impractical and were discarded; in 1879, Lady Mordaunt was reclassified as B class like Snake and they bore the classification for the remainder of their working lives.

[4] The two Bs proved unpopular with crews and failed to generate sufficient power, leading to their early withdrawal from service.,[5] Snake was retired in 1890 not long after receiving the number of B 238, while Lady Mordaunt lasted another six years.

[4] Three of the locomotives four power bogies were used under NZR-built steam cranes, nos 101-103.