Selwyn's distinctive approach was to learn Te Reo Māori (the Māori language) fluently and to spend extended periods traveling throughout the North and South Islands, visiting remote settlements, ordaining clergy, confirming believers, and understanding the needs of both European and Māori communities. He was a man of robust physical health and intellectual vigor who saw no reason to separate episcopal authority from pastoral presence. Contemporary accounts describe him tramping through forests, traveling by canoe, and sharing the hardships of frontier life alongside his clergy and people.

Between 1842 and 1868, Selwyn established the diocesan infrastructure: he ordained scores of priests (many trained initially through informal apprenticeship, later through a theological college), established schools including boarding academies, conducted visitations to every corner of the diocese, and created administrative structures that allowed the Church to grow sustainably. He advocated strongly for Māori education and ordination, supporting Māori priests and leaders—a position that was progressive for the 1840s-1860s, though the full story cannot be told apart from the pressures and complexities of the colonial era in which he worked.

In 1847-1848, Selwyn made a visitation to the South Seas beyond New Zealand, observing missionary work in Fiji, Samoa, and other islands. This experience led him to envision a more coordinated missionary strategy for the entire South Pacific. In the 1850s and 1860s, he extended episcopal oversight to include Melanesia (the New Hebrides, Solomon Islands, and adjacent territories), traveling extensively and founding what became the Melanesian Mission—a structured missionary endeavor with ordained clergy, schools, and sustainable local leadership.

The Melanesian Mission was marked by both missionary zeal and, importantly, respect for indigenous cultures. Selwyn learned languages, studied tribal customs, and insisted on training indigenous clergy rather than merely importing English clergy. This philosophy produced leaders like the Fijian priest and educator Frederic Colenso (and his associates), who sustained the mission's work after Selwyn's departure.

In 1868, at age 59, Selwyn relinquished the bishopric of New Zealand after 27 years and accepted appointment as Bishop of Lichfield (a major English cathedral see), where he remained until his death in 1878. His translation to England was partly due to his advancing age and partly because the New Zealand church had grown sufficiently to require different episcopal leadership; his choice to move rather than remain in place was characteristic of his obedience to the broader church's needs.

At Lichfield, Selwyn continued to exercise pastoral care, undertook visitations, and published theological writings. He remained engaged with the Melanesian Mission's work and was consulted on missionary strategy. He died on April 11, 1878, at age 69, having devoted his adult life to establishing Anglican Christianity in the South Pacific and preparing indigenous leadership to sustain it.