Te Ahu was a missionary among Te Arawa for over 50 years. From Te Uri Taniwha hapū of Ngāpuhi, he was born near Ōkaihau, in northern New Zealand. As a child, in 1832 or early 1833, he became a member of the household of Thomas Chapman, a Church Missionary Society lay missionary at Kerikeri. When Chapman established the first CMS mission station at Rotorua in 1835, Te Ahu was one of his assistants. Within a few years he married Rangirauaka, of Ngāti Riripō. On 9 May 1841 Te Ahu was baptised by A. N. Brown, the CMS missionary at Tauranga, and took the name Īhāia (Isaiah). At the same time Rangirauaka was baptised Katarina Hāpimana (Catherine Chapman). The couple were to have four daughters and one or more sons. Brown baptised one son, Pānapa, on 1 June 1841.
Between 1835 and 1845 the districts surrounding Rotorua were periodically disturbed by warfare between Te Arawa and the allied tribes of Waikato and Ngāi Te Rangi of Tauranga. Belonging to Ngāpuhi, who remained neutral in these conflicts, Te Ahu could safely accompany Chapman on his visits to the warring tribes. He soon became Chapman's principal mission teacher. By 1845 he was taking Sunday services when the missionary was indisposed. Chapman observed that Īhāia's ability was 'fully acknowledged around by all'. Īhāia and Rangirauaka, who lived at the mission, were also responsible for the management of Chapman's household. However, Īhāia, like Chapman, suffered from the cold climate at Rotorua, and in September 1846 he and his wife and two children moved to Maketū, in the Bay of Plenty. In 1851 the Chapmans, who visited Maketū each winter, settled permanently there.
Īhāia began his preparation for the Anglican ministry in 1857. From July to October, accompanied by his family, he stayed at the Tauranga mission station while studying under Brown. In the autumn and winter of 1858 he attended St Stephen's School, Auckland, to prepare for ordination. He took first place in a class examination on 29 April and won a Bible. But in August he returned to Maketū in poor health. There he continued mission work and in 1861 took charge of the station from Chapman, who was moving to Auckland. At Tauranga, on 3 November 1861, Īhāia was ordained deacon by Bishop William Williams.
Īhāia remained at Maketū through the wars of the 1860s. By 1867 the pressures of working without the support of other missionaries and the continuing Hauhau unrest had left Īhāia pessimistic about the future. Īhāia had built his own house in Maketū pa and lived there with his wife; there he was visited by the Reverend T. S. Grace early in 1868. Grace said prayers with Īhāia, but when he asked him to ring the bell to summon the congregation to service, Īhāia predicted that the people would not come. Nevertheless he was committed to the Maketū mission and his work resulted in the building of the church of St Thomas, which was opened on 21 March 1869 and still stands.
The arrival of the Reverend S. M. Spencer and his family at Maketū in 1870 fulfilled Īhāia's desire for a strong European missionary on the Bay of Plenty coast. It also enabled him to spend more time at Rotorua and to visit the eastern Bay of Plenty. Īhāia was not satisfied with the progress of Christianity, however. Brown told the CMS in 1873: 'His reports are of a sad character as to the spiritual state of the natives although they pay him marked respect and listen attentively to his preaching while making these periodical journeys.'
By 1881 Īhāia Te Ahu was living permanently in Rotorua. In 1882 he became the first vicar of the Ōhinemutu pastorate and began a drive to build a church. St Faith's Church, Ōhinemutu, was consecrated by Bishop E. C. Stuart on 15 March 1885. Stuart paid tribute to the zeal with which Īhāia had worked since his ordination 20 years earlier. Īhāia had become known as the 'hero of missionary effort' in the Rotorua area. He left Ōhinemutu in 1889 and, after serving at St Stephen's College, Auckland, had retired by 1892. He moved to Kaikohe, where he died on 7 July 1895.