William Downie Stewart was born in Dunedin, New Zealand, on 29 July 1878, the fifth child of the lawyer and MHR William Downie Stewart and his wife, Rachel Hepburn, daughter of George Hepburn, an early merchant, politician and elder of Knox Church. His mother died within months of his birth and his father remarried in 1881. The children were brought up by a succession of nannies and nurses in an atmosphere of stern Calvinistic Presbyterianism and politics. Downie Stewart senior had a particular interest in social reform, which they all inherited, and the Stewart household was a centre for political and religious gatherings.
Known to his family as Willie, but commonly known to political colleagues as Downie, Stewart was educated at Otago Boys' High School from 1888 to 1894 and at the University of Otago. He was prominent in student affairs, being a member of the debating society, treasurer of the students' association in 1898 and editor of the student newspaper Otago University Review from 1899 to 1900. On the death of his father in 1898 he was employed as a law clerk in his legal firm. He became a partner on graduating LLB in 1900.
Stewart stood for Dunedin South in the general election of 1905, surprising many who knew him by running as an opposition candidate. He later explained that he opposed the Liberal government's lease-in-perpetuity scheme, and he was never an admirer of Richard Seddon, whose autocratic style of government appalled him. After his defeat he turned his attention to local government, becoming a city councillor in 1907 and mayor of Dunedin in 1913.
Stewart remained uncertain as to his political allegiance. He could not adapt himself to the cut and thrust of party politics. To Stewart, Parliament was ideally a meeting of minds where decisions were made for the common good and where debate should be an intellectual rather than a political exercise. He had a genuine interest in social reform and an academic interest in socialism, but the labour unrest before the First World War drove him to join the Reform Party. It is ironic that he had more in common and felt more at ease with Labour MPs than with his own colleagues, and was to form deep friendships with the leading Labour politicians Harry Holland and John A. Lee.
In 1910 State socialism in New Zealand, a critical analysis of the Liberal government, was published. Stewart co-authored it with James Le Rossignol, professor of economics at the University of Denver in the United States. The circumstances surrounding the collaboration remain vague, but the book did much to establish Stewart as a scholar both nationally and internationally. Subsequently, in 1914, he wrote the preface to the English edition of André Siegfried's Democracy in New Zealand.
Downie Stewart agreed to stand in Dunedin West for Reform in 1914, but with the advent of war asked to be released so that he could enlist. William Massey, the Reform prime minister, urged him to wait until after the election, promising that if he had a sufficient majority he could be released for war service. Stewart was elected in December and he and the Liberal politician T. E. Y. Seddon subsequently paired and enlisted as privates in a bid to encourage recruiting. Commissioned as a second lieutenant in the Otago Infantry Regiment in November 1915, Stewart left for Egypt in January 1916. While there he suffered from what was thought to be an attack of rheumatism. He recovered and went with the 1st Otago Battalion to France, but the rheumatoid arthritis which was to cripple him for the rest of his life recurred. Invalided to England in August, he returned to New Zealand in December 1916.
Stewart received a warm welcome back to the House of Representatives in June 1917 when he entered on crutches supported by two attendants. This return to public life earned him the respect and affection not only of his colleagues, but of the press and the public. He was initially reluctant to stand in 1919, but Massey persuaded him to do so, offering him cabinet ranking in 1920. He declined at first on health grounds, but in 1921 accepted the portfolios of internal affairs and customs and in 1923 industries and commerce. In 1926 he was acting prime minister for a brief period.
While Stewart's career flourished his health deteriorated. By 1925 he was confined to a wheelchair. He travelled to New York to seek further treatment and was absent when Massey died on 10 May 1925. The Reform Party, faced with a general election, chose Gordon Coates as his successor. Contemporary opinion considered that Stewart had been betrayed, with the decision being made in his absence, but it is unlikely that Stewart's health would have enabled him to undertake the premiership. In 1926 he withdrew from the office of attorney general as he thought it was a portfolio that needed legs. He never sought to become prime minister and seemed confused at the level of support that he received.
William Downie Stewart was a careful and rational administrator rather than a vigorous policy maker. As minister of customs he negotiated trade agreements with Australia, Japan and Belgium. As minister of finance from 1926 to 1928 he oversaw every measure of the Reform government; Coates referred to him only half-jokingly as 'almost a dictator'. With the defeat of the Reform government in 1928 Stewart advocated the coalition of the Reform and United parties. When this occurred in 1931, he resumed the finance portfolio.
Coates and Stewart headed the New Zealand delegation to the Imperial Economic Conference in Ottawa in 1932. Much was expected of this conference and Stewart throve as one of a coterie of academic imperialists. He was actively sought out by other delegates and invariably held court in his pyjamas before sessions. He regarded the conference as a success for New Zealand, in as much as a guaranteed market in Britain was gained for five years on very favourable terms. Afterwards Stewart, accompanied by A. D. Park, secretary to the treasury, visited London for discussions with leading financiers. On advice received from the Bank of England he introduced and saw through its first reading a Reserve Bank of New Zealand Bill in December 1932.
Stewart disagreed with the measures his colleagues considered necessary to alleviate the depression. It became increasingly clear that he was the odd man out in a cabinet concerned to ameliorate the lot of the farmer: Stewart had little sympathy for their plight, which he believed was to a large extent self-inflicted. He contended that New Zealand would have to trade its way out of difficulty, relying on the goodwill of creditors. He was particularly concerned with measures in the National Expenditure Adjustment Bill 1932, which proposed to reduce the internal rate of interest paid by the government. Stewart viewed this as a breach of contract between the government and its creditors. As minister of finance he declined to accept responsibility for the bill and it was presented to the House by George Forbes. When the government opted to devalue the New Zealand pound, Stewart maintained that this was a theoretical breach of the Ottawa agreement, which had guaranteed the status quo for five years; he opposed all artificial tampering with the rate. Unable to support the measure while responsible as minister of finance for its passage and implementation, he resigned from cabinet in January 1933.
Although hailed as a calamity in most sections of the community, his decision caused him little agitation. Of unimpeachable integrity, he escaped the ignominy incurred by Forbes and Coates, and despite becoming the focus of opposition to the government's anti-depression policies, he continued to support the coalition. He was defeated in the 1935 election.
On his retirement he recommenced his writing career. He had edited the Journal of George Hepburn for publication in 1934, and the success of this volume encouraged him to write about men such as Sir Francis H. D. Bell, William Rolleston and Sir Joshua Strange Williams. He produced Mr Justice Richmond and the Taranaki War of 1860 and a brief history of the Dunedin Club. In addition to these published works he prepared a history of the Labour Party and an autobiography entitled 'Three generations in New Zealand politics', which included details about the careers of both his father and grandfather.
Stewart also busied himself with a number of organisations. His concern at the lack of knowledge in international and empire affairs shown generally by New Zealanders led him to become involved in the New Zealand Institute of International Affairs and the Round Table; he continued to represent New Zealand at international conferences of these organisations until 1938. From 1912 to 1919 he had represented the Dunedin City Council on the Council of the University of Otago, and from 1943 to 1948 he held a position on the Senate of the University of New Zealand. However, travelling from Dunedin to Auckland to attend a meeting was 'a nightmare', and although he subsequently rejoined the Otago university council he declined the offer of the vice chancellorship. His interest in community welfare was evinced by his continued support for the Patients' and Prisoners' Aid Society and the Plunket Society.
Despite his legal training Stewart had no great love of the business world and he took only a cursory interest in politics after 1935. He held directorships in several prominent firms such as the Westport Coal Company and the National Insurance Company of New Zealand. The founders of the New Zealand National Party were eager to obtain his support but he was extremely reluctant to join; he questioned their motives. Although a long-term advocate of the merger of the Liberal and Reform parties, he had never advocated their fusion solely to fight socialism, which he considered the philosophy of the future. He also felt that, as a representative of the old order, he had no place in the new party. He was eventually persuaded to join, but played no active role.
His health deteriorated and a stroke in 1947 left him partially paralysed. He subsequently taught himself to write with his left hand. William Downie Stewart died at his home on 29 September 1949. He had never married, and was supported and cared for throughout his career by his sister, Mary Downie Stewart. He was buried from Knox Church, where he had been an elder, and his remains were interred in the family grave in Dunedin's Southern cemetery.