Emma Brignell Roberts was born in Westham, Essex, England, probably in 1848 or 1849, the daughter of Mary Griffith and her husband, Thomas Roberts, a clergyman. In 1852 her parents emigrated to Melbourne, Australia, with their 11 children.
On 14 July 1868 at Melbourne, Emma Roberts married William Henry Ostler, runholder at Benmore sheep station, North Otago, New Zealand, where the couple settled. Their daughter Helen Mary was born in 1869, Edith Louisa (Daisy) in 1874, and their son, Henry Hubert, in 1876.
In 1874 William Ostler bought Ben Ohau station in South Canterbury. When he died on 11 May 1879 the mortgagees foreclosed, leaving Emma without even the furniture from her home. Although £1,000 came from her husband's life insurance, she set this aside for her children's education, a matter of importance to her. Helen (whose married name was Wilson) later wrote a book, My first eighty years, and Henry became a Supreme Court judge.
Well-read, and interested in the physical sciences, particularly geology (a small hammer and chisel for examining rocks were always taken on excursions with her children), Emma Ostler despised her own education at a Melbourne school for young ladies. Nevertheless, she drew on skills learned there to earn a living: after moving to Timaru she taught dancing, painting and fancywork. In 1887, with her two youngest children at school overseas, Emma Ostler moved with her daughter Helen to Waitohi Flat where Helen had obtained a teaching position. Here Emma developed 'an acute attack of land-hunger'. In 1888 they moved north to take part in the ballot for village settlement sections at Levin, securing 20 acres with permanent water. Together they cleared the land, gardened, fenced, and, observing how their first one-roomed shelter was constructed, built their own additions. Their property, Cashmere, when added to over the years, became a valuable estate. During her years in Levin Emma revealed a shrewd business sense, making her fortune in leases and real estate. She is known to have annoyed the local county council by leasing a gravel reserve for grazing then selling the gravel, and also by renting probably the same land to horse trainers.
Uninspired by her religious upbringing, Emma Ostler was otherwise a woman of strong convictions; homoeopathy was one of them. A supporter of women's suffrage, she knocked on doors and gathered signatures for petitions. Like her father, a temperance advocate, she became 'a rabid teetotaller'. She was an officer of the Levin branch of the New Zealand Women's Christian Temperance Union. From 1898 until about 1918 she was a vice president of the New Zealand Alliance, which campaigned for the abolition of liquor traffic. During this period she became closely associated with Sir Robert and Lady Anna Stout.
By 1908 Emma Ostler and her daughter Daisy were living in Wellington. Around 1914 they moved to Remuera, Auckland, where Emma died on 14 April 1922. Emma Ostler was described as 'Calm, deliberate, sincere, courageous, and intelligent'; her friend, Sir Robert Stout, referred to her as 'one of New Zealand's great pioneers'.