Thomas Forrester was born in Glasgow, Scotland, probably on 16 May 1838, the son of Janet Watt and her husband, George Forrester, a modelling and decorative plasterer. Thomas attended the Glasgow Government School of Art, where his studies ranged from map drawing to the draughting of plans. Meanwhile, he learned from his father how to ornament the interiors of buildings, and like him became a plasterer. On 5 July 1860 at Glasgow, Thomas Forrester married Elizabeth Megget, a dressmaker. The following year Thomas and Elizabeth and the first of their four children travelled to New Zealand as assisted emigrants, arriving in Port Chalmers, Otago, on the Pladda on 8 September 1861. Thomas Forrester's parents accompanied them to New Zealand.
During his first years in the colony, Forrester was called on to use the full range of his artistic talents. Initially he is said to have worked as a plastering contractor in Dunedin. In 1865 he drew for James Hector the first geological survey map of New Zealand. The same year the architect William Mason designed the structure for the Dunedin-based New Zealand Exhibition. Forrester, who had worked as a draughtsman to Mason and his partner W. H. Clayton, was superintendent of the building where displays were shown.
After working for Mason and Clayton, Forrester was employed by the architect R. A. Lawson. In early years Oamaru limestone was much favoured as a building material, large blocks being available 'of the same tint and consistency' so that 'whole cities might be built in which one stone could not be distinguished from another'. Mason had used the easily worked material for the magnificent, ill-fated Dunedin post office, completed in 1868; a more secure edifice was Lawson's limestone-veneered First Church of Otago, begun the same year. Thus in 1869, when Forrester was sent to Oamaru as supervising architect for Lawson's Bank of Otago building (later the National Bank of New Zealand offices), he well knew the material with which he was working.
His assignment completed, Forrester settled with his family at Oamaru. He soon began a lifelong connection with the local harbour authority as inspector of works, secretary and, later, engineer. He established that the seafloor could be dredged, and that a notoriously uncertain open roadstead could be transformed into an enclosed deep-water harbour. About 1872 he and John Lemon formed an architectural partnership: Forrester was part-time designer of buildings, while Lemon, with his well-established contacts among the local élite, was full-time business manager.
Occasionally the practice took on contracts outside North Otago. After the abolition of the provincial system and establishment of a multitude of counties in 1876, the Waimate County Council commissioned Forrester to design council chambers and a hospital. The brick and stucco chambers, among the first in New Zealand, had spacious and comfortable debating facilities. The hospital plans were considered more suited to 'a place of affluence than…charity', and the completed brick building was later criticised as a memento of a time when local bodies had more money than they could usefully spend.
In Oamaru, by contrast, it was thought that structures should be imposing and that a utilitarian function should be concealed behind an ornamental facade. Forrester, deferring to local expectation, created on Harbour Street an uninterrupted sequence of grand buildings constructed in Oamaru stone in which grain was stored before being moved to the nearby port. In neighbouring Tyne Street, the town's original business heart, there arose richly ornamented hotels, the squat Custom House, and J. & T. Meek's Elevator, a five-storey grain store. For Thames Street, which became Oamaru's main artery, Forrester planned a court house which, with its classical design, emphatic temple-front portico and restrained facade, was the pride of the district when completed in 1883. Also in Thames Street was the large, decorated post office finished in 1884; it was for years a favourite with postcard, calendar and crockery illustrators. The freezing works, which took sheep from the nearby Totara estate, was completed in 1886. Beyond municipal boundaries, the partners designed the original Waitaki Boys' High School building with its picturesque silhouette.
When John Lemon died in 1890, Thomas Forrester retired from the business to make way for his only surviving child, John Megget Forrester. Thereafter he pursued his interests in geology and photography. With Dr H. A. de Lautour he did microscopic work of scientific value on the diatomaceous deposits of the Oamaru district. Forrester continued to focus much of his attention on harbour improvements. He designed the Holmes wharf which allowed large ocean-going vessels to call at Oamaru. A festive crowd greeted the 6,237-ton Waiwera in June 1907. Sadly, Forrester did not live to witness this occasion, having died in Oamaru on 25 March 1907, survived by his wife and son.
In colonial New Zealand a man who had not been articled to an architect might still break into the profession. Forrester was one such man, progressing from plasterer, to draughtsman in the employ of Dunedin architects, to independent practitioner in Oamaru. Local dignitaries, encouraged by the illusory prosperity of the Vogel years, insisted that the churches, schools, warehouses, factories and private houses of their town be of a high architectural standard. Forrester obliged. Working with local limestone, he erected handsome buildings, many of which still stand, monuments to his skill and to North Otago pride.