Together with Mary Ann Martin and Caroline Abraham, Sarah Selwyn contributed her private correspondence for a publication protesting at the British colonial policies of land confiscation and military conquests against the Māori in New Zealand.

She spent much of her childhood in London, but, due to his asthma, her father, Sir John Richardson (1771–1841) retired from his service as judge of the Court of Common Pleas.

[2]: 7  Lady Harriet died in March 1839, a few months before Sarah was to be wed.[2]: 12 At nearly thirty years of age, she married George Augustus Selwyn on 25 June 1839, at St Giles in the Fields, London.

[4]: 29 When Canon William Selwyn turned down the appointment for a new bishopric for New Zealand,[5][2]{rp|13} the Archbishop offered it to his younger brother George who accepted it in July 1841.

Sarah Selwyn's father, Sir John Richardson, died earlier that year at his house in Bedford Square, London, on 19 March 1841.

There Bishop George Selwyn established St John's College using his residence and adjacent buildings to train candidates for ordination into the Anglican Church.

When the Bishop expanded St Stephen's to serve as a theological school for male Māori, Margaret was supported by Mary Ann Martin and Sarah Selwyn as she took on the training of the candidates' wives and children.

Caroline Abraham wrote that "Sasa" (her nickname for her cousin Sarah Selwyn) "used to shut herself up in her room & come out only to slave away at some drudgry, or some teaching work, & look distressed & one dared not notice it, least she shd.

In 1859 groups of Māori led by Wiremu Kīngi Te Rangitāke opposed the colonial government's attempts to purchase land near the Waitara River in north Taranaki.

Several prominent citizens and missionaries published formal public statements in newspapers and pamphlets protesting against those politicians and settlers pushing for a military solution and uncontested access to the rich farming land held by collective groups of Māori.

While their husbands used newspapers, public speeches and books for their writings about the events at Taranaki, the women expressed their views in personal letters to frame the issue on moral grounds.

[15] In defending her husband's objection to the Governor's actions, Sarah's private letter to her cousin Mary Anne Palmer, a sister of Caroline Abraham, (dated 30 August 1860)[16]: 121–22  started with "I must write a pamphlet, or I shall burst."

She argued that the colonial government, the governor, the military leaders and white settlers, all had: Sarah Selwyn left New Zealand on the troopship Boanerges on 5 February 1861 to visit her sons at Eton.

Even though Sarah Selwyn was part of the colonising wave of the Pākehā through her mission work, she protested at what she considered an illegal confiscation of Māori land.

She also, in a radical way compared to other English missionary wives, challenged the growing sentiment that white settlers had an inherent superiority over the indigenous people of New Zealand.

[17] Even later in life, in her Reminiscences written in the 1890s for her children and friends, Sarah Selwyn had sharp words for "the blindness and wrong-headedness of the Ministry" during this time of war.

Eventually, however, he did accept after an intervention from the Queen on condition that they be allowed to return to New Zealand in order to say their goodbyes to the country they had called home for the previous 26 years.

"[7]: 73 Although suffering from ill-health her whole life, she outlived her son John (died 1898), who had like his father become Bishop of Melanesia and, from 1892 to 1895, had served as honorary chaplain to Queen Victoria.