Mary Ann Barnes was born in London, England, on 20 February 1836, the daughter of Susan Barnes and her husband, John Thomas Barnes, a carpenter. She arrived in New Zealand in 1859. At Onehunga on 9 May 1861, aged 25, she married Thomas Caesar Colclough, a farmer of 54 or 55, first in St Peter's Anglican Church then in the Catholic church. A daughter, Mary Louise, was born at Papatoetoe on 1 November 1862, and a son, William Caesar Sarsfield, was born at Otahuhu on 26 January 1864. Thomas Colclough died in the provincial hospital, Auckland, on 29 July 1867. As a well-qualified and experienced teacher Mary Colclough supported the family, running her own school for girls in Auckland from 1871 to 1872, and teaching at Tuakau from September 1872 and Kauaeranga from July 1873 on a salary of £200 a year. By late 1874 she had left for Melbourne, Australia.
To Aucklanders in the early 1870s Mary Colclough became a household name. As 'Polly Plum' she carried on an exhaustive public correspondence in the local press, taking on all comers. She gave public lectures in Auckland, Thames, Ngaruawahia and Hamilton – an extraordinary activity for a woman at that time. She advocated temperance and improved treatment of women prisoners and prostitutes, becoming involved in their practical rehabilitation. Above all, she championed women's rights.
Influenced by the writings of John Stuart Mill, Mary Colclough targeted the contemporary legal position of married women in New Zealand: they had no independent legal status, and no control over property or guardianship of their children. It was 'iniquitous', she argued, 'that in a Christian country, anyone, male or female, should have it in their power to wrong and oppress others, under the shelter of the law'. She spoke from bitter personal experience: because of her own husband's ruinous speculation with her earnings, bailiffs had stripped the family residence down to bare floorboards. 'I was the breadwinner,' she wrote, 'whilst he had all the breadwinner's powers and privileges.'
She argued that women were entitled to education, careers and the vote. In her opinion the role of wife and mother was very important but it was absurd to educate girls purely for domestic life. As single women, widows, or wives of improvident husbands, many would have to support themselves. Self-reliance and self-help were the keys. There should be no legal barrier to women rising as high in the world as their talents would take them. Finally, justice demanded that women should not be subject to laws they had no part in making.
Aucklanders, both men and women, reacted with outrage or applause. Opponents ridiculed and patronised her, charged her with impropriety of conduct, and condemned her on biblical grounds. But many cheered. Her lectures drew enthusiastic audiences and women supporters were surprisingly outspoken. All parties acknowledged her talent. The Reverend Samuel Edger, a remarkable non-denominational minister and father of Kate Edger, the colony's first woman university graduate, was among her strongest advocates.
In late 1874, while in Melbourne, an even more radical Mary Colclough attacked the institution of marriage itself and challenged citizens to dispute with her on a public platform. The Australian press was harsh and vociferous in its opposition to her ideas. Afterwards she returned to New Zealand, but from then on disappeared from public view. At first she taught in Auckland. She then moved to Canterbury, where from 1876 to 1878 she was headmistress of Rangiora Girls' School, and in 1881, infant mistress at Papanui. She died in Picton on 7 March 1885, aged 49, a month after fracturing a leg and an arm in an accident. She was survived by her two children. Mary Colclough was a highly controversial public figure for a few years only, but she jolted the people of Auckland by fundamentally challenging contemporary assumptions and values about woman's place in New Zealand society.