The NZR of A class of 1873 consisted of three types of similar specification but differing detail. The first and most numerous from Dubs and Yorkshire Engine Co., the next from the Wellington firm E.W. Mills Lion Foundry, and the last from the Scottish firm of Shanks. The specifications are for the Dubs/ Yorkshire engines.
The A class was the second class of steam locomotive (after 1872's F class) ordered to work on New Zealand's national railways. Initially ordered by the Public Works Department for use in the construction of lines in Cantebury and Taranaki, the A class was a small tank locomotive with a wheel arrangement of 0-4-0T. An initial twelve were constructed by Dübs & Co. in 1873 and two more were built in 1875 by Yorkshire Engine Company. They were not just used by the Public Works Department; the New Zealand Government Railways also utilised the class to operate revenue services on smaller branch lines. Quickly outmoded for use on the lines they helped build, only one (A62) remained in government service by 1905, used on the NZR's Stores Branch tram line at Piha. Their small size made them perfect for use on bush tramways and small private industrial sidings. Many members of the class survived for decades in private use, and although all are now retired from commercial service, four have survived to be preserved by railway enthusiasts and two of the four are currently in full operational condition. One of these preserved locomotives, A 67, was the first in a cavalcade of locomotives at the celebration of the hundredth birthday of the Dunedin Railway Station. 
A batch of A class engines, with differing external details was built at Wellington's Lion Foundry, by E.W. Mills Ltd in 1873, for use on the Foxton Section. These appear to be the first NZR locomotives actually built in the country. Like other so-called contractors engines they were quickly outmoded for line haulage and were sold to industrial operators. "Opossum" was sold in 1877 and served industrial and timber companies for eight decades. The other two were likewise sold by 1885.
A further batch was built by Alexander Shanks and Company, of Arbroath, Scotland in 1876 on behalf of the Otago Provincial Council for construction work on the Riverton and Otautau lines in Southland. Both were sold by 1882 and one, Mouse spent the next four decades in the local timber industry. The other went to similar work in Westland.
The term "Dido", as applying to New Zealand shunting locomotives, can be traced back to the small A class. Crew members from the ship Dido, moored at Bluff Harbour, in May, 1875 were looking for a night on the town, while in port. They took a small 4-wheeled rail trolley, and taking turns, both rode and pushed the trolley into Invercargill. When the southern enginemen saw the first A class locomotive arrive for use as a shunter, the name "Dido" was given as a nickname because of its diminutive size. This size led British Journalist Charles Rous-Marten to describe them as "a most absurd dwarf". Today, the term "Dido" is used to describe any small shunter on the NZR network.