Around the turn of the century, Dufaur made his former law clerk, Charles Edward MacCormick (1862-1945) his partner. MacCormick also specialised in Maori land. In 1906 he was appointed a judge of the court then known as the Native Land Court and in 1940 Chief Justice of that court. Being appointed to the bench meant MacCormick had to retire from the firm: the practice was transferred to Noel Biss (1870-1925), and became known as Dufaur & Biss. In 1912 the firm amalgamated with another Auckland firm whose partners were Robert Butler Lusk (a prominent sportsman who had played football for Auckland, Taranaki and New South Wales) and Robert Edward Fawcett (1888-1954).
One of the employees of the amalgamated firm – Dufaur Lusk Biss & Fawcett – was Percy Dufaur (1861-1944), a younger brother of the firm's founder. He never qualified as a lawyer but he was something of an institution in the Auckland legal world despite, by all accounts, occasionally exasperating his employers.
Following the Biss, Lusk and Percy Dufaur era, the firm was carried on by the "energetic and versatile" RE Fawcett who, although not himself a leftist, went on to represent a number of unions including the Auckland branch of the Waterside Workers' Union which fomented the historic 1951 waterfront strike. Fawcett was joined, soon after the end of the Second World War, by GE (Jock) Cairns who had returned from the war a Brigade Major bearing the Military Cross. In 1957 Cairns was beginning to suffer ill-health and brought in as a partner the newly-qualified Bruce Slane.
The 1960s saw the firm expand rapidly and the retirement of Jock Cairns was followed by the admission as partners of John Phillips, Robert Ludbrook and Gary Judd (now an Auckland QC). At the beginning of the decade the firm, like most others in Auckland, was a general practice; by the end of it the firm had developed specialties in domestic proceedings work and commercial, administrative and other litigation. Bruce Slane went on become President of the New Zealand Law Society and, eventually, Privacy Commissioner.
The firm was called Cairns Slane Phillips from 1964, but in 1988 this was formally abbreviated to what people usually called it anyway – Cairns Slane.