This steel-framed structure was one of Auckland's first 'high-rise' office blocks and marks an important transition between Victorian commercial structures and modern office buildings. It was built during the First World War as the headquarters of the New Zealand Insurance Company (NZI), which had been founded by local businessmen in 1859. The NZI specialised in marine and fire insurance, and had branches throughout the British Empire, America and the Far East. By the early 1900s most of the firm's profits came from its overseas operations, and it began to rebuild many of its offices from 1909, initially in Australia and then in New Zealand.

The eight-storey structure housed head and branch office accommodation, as well a further 137 offices for let to other firms. It replaced a three-storey NZI building on the site, which had itself been considered an imposing Queen Street landmark. Designed by William Gummer, the new structure was modelled on turn-of-the-century commercial buildings in America and elsewhere, and was of Stripped Classical style. Stripped Classical adapted nineteenth-century classical architecture to the requirements of twentieth-century office blocks, which included a need for height and a large amount of natural light. The size of the new headquarters dramatically exceeded most other office structures in the city, enabling the company to project an image of solidity and wealth while generating a long-term income from rent. The building, which incorporates a clock from the earlier NZI structure on the site, was refurbished in the 1960s. After the NZI merged with the South British Insurance Company in 1982, it was renamed the New Zealand Guardian Trust Building and extended with an additional storey.

The New Zealand Guardian Trust Building is significant as the earliest remaining high-rise office block in Auckland, reflecting important changes in commercial design and the organisation of business in early twentieth-century New Zealand. It is important for its associations with the NZI, which was one of the first public companies in the country to be floated using local finance. The structure is an early example of a Stripped Classical design, and the earliest major building by William Gummer, one of the most influential architects in early twentieth-century New Zealand. It has important landmark qualities and makes a significant contribution to the Queen Street streetscape. It is valuable as one of a group of historic buildings in the Queen Street and Shortland Street area that show successive changes in commercial design and organisation from the nineteenth to the early twentieth century.