Henry Hubert Ostler was born at Ben Ohau station near Timaru, South Canterbury, on 2 July 1876, the third child and only son of Emma Brignell Roberts and her husband, William Henry Ostler, the station proprietor. After William's sudden death, when Hubert was not yet three years old, Emma was for financial reasons forced to leave the farm, and with her young family moved to Timaru. In 1886, on the recommendation of a relative, Hubert won a nomination to Christ's Hospital school in London, where some of the Ostler family had been pupils. He remained in England for six years. A career at Cambridge University would have been open to him, but he returned to New Zealand to assist his mother, who had been successful in a ballot for a village settlement section of 20 acres near Levin. For nine years he, his mother and his sister Helen cleared and developed the land.
On the outbreak of hostilities in South Africa in 1899 Ostler was keen to join the New Zealand contingent, but felt it his duty to help his mother run the farm. He inquired about enlistment the following year after the farm had been leased, but on the advice of Sir Robert Stout, a fellow worker for prohibition with Ostler's mother, he was persuaded at the age of 24 to begin part-time law studies at Victoria College. He completed the matriculation examination in January 1901, gaining first place in New Zealand. To finance his studies Ostler worked in a sawmill during the summer months and, after many applications to Wellington law firms, succeeded in obtaining employment as a law clerk with Alexander Dunn. His law studies were completed in 1904 and he graduated LLB the following year. From 1905 to 1907 he enrolled for an LLM degree, but each year pressure of work prevented him from continuing.
At the university he threw himself enthusiastically into the life of the student community. He was founder and first captain of the Victoria College Rugby Football Club (1903–5) and was a Wellington provincial representative rugby player. He was also treasurer of the Victoria College Students' Association in 1902, a university tournament delegate in 1903, and the founder and first editor of the Spike, the college magazine. After his graduation he maintained a close interest in the well-being of the college, serving as a member of the College Council in 1911 and as chairman in 1913–14. From 1915 to 1919 he was a member of the Senate of the University of New Zealand.
On completing his law studies Ostler was admitted as a barrister and solicitor on 6 May 1905, having in March 1903 become an associate to Stout, then chief justice. Ostler's duties introduced him to the judges of the Supreme Court and gave him a familiarity with court procedures, as well as providing opportunities to see advocates in action.
In July 1906 Ostler became the editor of the New Zealand Law Reports and held that position for the next four years while practising as a barrister. Early the following year he resigned his associateship to embark on practice as a barrister only – a most unusual course for a law practitioner at the time. He was fortunate to attract the favourable attention of Francis Bell and Charles Skerrett, who had recently been appointed King's counsel. He frequently appeared as their junior counsel and also devilled for them in cases where their legal opinion had been sought. Through a combination of natural industry and concentrated application, Ostler quickly acquired a reputation for detail, accuracy of recollection and mastery of facts.
Ostler married Lucilla Bernice Duigan at her mother's home in Wanganui on 7 January 1911. There were three children of the marriage: two daughters and a son. In 1910 John Salmond, the solicitor general, offered Ostler the position of first assistant law officer in the Crown Law Office. Although the salary offered was less than half what he was then earning, he was anxious to enlarge his experience of legal practice, particularly as the position carried with it appointment as a Crown solicitor and Crown prosecutor. Ostler's five years as a law officer gave him invaluable experience in a wide range of legal work.
In 1915 Ostler was invited to become a partner in the Auckland firm of Jackson and Russell, and for the next five years he was heavily engaged in a demanding litigation practice. The unremitting pressure of work began to take its toll, and when the opportunity arose in 1924 he accepted the offer of appointment to the Supreme Court Bench on condition that he would not be expected to commence duties until the following year and that he would first be appointed a King's counsel. In fact, in a most unusual sequence of events, on the same day (2 February 1925) as he was appointed King's counsel, Ostler was appointed a judge.
On the Bench, Ostler was a hard worker, impatient of perceived technicalities, open to argument, never overbearing but inclined to be somewhat swift in judgement, possibly because of an absolute rule to deliver judgements promptly. He was sound and workmanlike, although few of his judgements are regarded as outstanding contributions to jurisprudence. During this period Ostler was appointed president of the Prisons Board; in this capacity he became involved in a wounding public controversy on prison reform with Frederick de la Mare, an old friend and professional colleague.
Ostler's judicial career was blighted by a slow, progressive illness which he attributed to an accident in his farming days but whose symptoms became apparent only three years after his appointment to the Bench. Because of the loss of grip in his right arm and fingers, he was forced to learn to write with his left hand. Nevertheless, he was a lover of the outdoors, a keen fisherman, and an ardent hunter and traveller. A typical photograph shows him as a hunter, pipe in mouth, a brace or more of game slung over his shoulder, a man of disciplined physical vigour, who believed in living dangerously. He was a life member of the Wellington Acclimatisation Society and wrote articles for Blackwoods Magazine on whale chasing in Cook Strait and on game-hunting in Africa. He had a lifelong fascination with Africa and an interest in a property in Northern Rhodesia (Zambia). In 1935 he embarked upon a strenuous vacation, driving a car for 16,000 miles from Cape Town through central and east Africa and back again. In 1937 he wrote articles in the National Review criticising government and judicial administration in central and east African Crown colonies.
Ostler was a courageous man of enormous energy. He was tolerant of human weakness and frailty, intolerant only of injustice, insincerity and hypocrisy. He held strong views on many questions, even to the point of unconsciously alienating old friends. He was knighted in 1939.
The cause of Ostler's steadily advancing paralysis was diagnosed to be cancer of the spinal cord; with increasing pain that could not be assuaged by surgery, he was forced to resign on 1 February 1943. He lived for another year, dying in Dunedin on 24 February 1944. He was survived by his wife and children.