The Railway Bridge and Viaduct at Mechanics Bay, Auckland is believed to be the earliest bridge linked with steam railways to survive in the North Island. Originally constructed in 1865-1866, it formed part of the ill-fated Auckland to Drury Railway project, which proposed to link the growing colonial settlement at Auckland with sources of coal in the Waikato. The route was also intended to provide a supply line to British military forces and early settlers after the colonial invasion of the Waikato in 1863-1864. The first part of the project involved constructing a length of track between the industrial and commercial suburb of Mechanics Bay with the outlying settlement at Newmarket. The works were to include a stone bridge and viaduct at Mechanics Bay, as well as cuttings and a tunnel beneath the Auckland Domain. In an attempt to save money, the contract was offered to a local scoria carter, Peter Grace, who had little previous experience of undertakings on this scale. The overall project was the largest carried out by the Auckland Provincial Council by that time.

Probably designed by the main project engineer, James Stewart, in association with Samuel Harding, the structure consisted of a bowstring bridge and a viaduct over Parnell Rise, all resting on large basalt piers. Carried out in a monumental style for the period, the ashlar masonry was mortar-bonded, concealing a rubble backing. Work on the structure was probably finished in October 1866, by which time overruns on time and budget had forced the rest of the project to be cancelled. Not having been connected to central Auckland across the waters of Mechanics Bay, the structure remained unused as an expensive white elephant until the public works policies of Julius Vogel (1835-1899) in the 1870s caused the Bay to be reclaimed and a line from Auckland to Onehunga completed. Small modifications were made to the bridge at this time. When completed, the line was the first of Vogel's railways to be operational and the earliest to be run directly by the General Government.

Slight alterations were made to the bridge in the 1890s, when some of the masonry piers were raised in height. More substantial modifications occurred in 1908-1909, at which time the entire superstructure was replaced. The new superstructure incorporated a bridge of Warren truss type, while two of the piers beside Parnell Rise were removed to enable the road to be enlarged. The remaining piers and abutments were widened using an almost identical style of construction to the 1860s work. The works allowed for the installation of a wider, double-track line, and were completed at the same time that the main trunk line between Auckland and Wellington was opened in 1909. The structure has been comparatively little altered since that date and remains in daily use for rail transport, although no longer part of the main trunk line. The structure is considered to be important for its associations with New Zealand's transport and technological history, and has architectural and aesthetic qualities for its monumental nature and use of local materials, notably basalt. It also has archaeological value, retaining nineteenth-century fabric in its piers and abutments, and other archaeological material in its embankments.