Arihia Kane Tāmati, who was to become Arihia Ngata, was born at Whareponga on the East Coast of the North Island, according to family information in 1879. She was the fourth of nine children of Mere Arihi Kākano and her husband, Tuta Tāmati, who belonged to Te Aitanga-a-Mate hapū of Ngāti Porou. She also had connections through her father with Te Whānau-a-Rākairoa and Te Aowera hapū. The Tāmati family owned and operated a hotel. Tuta Tāmati was an associate of Paratene Ngata and was a foundation member of the Polynesian Society. Arihia is believed to have attended a local native school.
Arihia's parents were traditionalists who adhered to the chiefly practice of selecting spouses for their children. Arihia's elder sister, Te Rina, was betrothed to Apirana Turupa Ngata, the son of Paratene Ngata, but she died before the marriage could take place. According to custom the elders consoled the groom by marrying him to a younger sister. Arihia was 16 at the time of her marriage to Apirana Ngata in 1895; she went to live in Auckland where he was working for a law firm while completing his university studies.
Arihia had 15 children, of whom three boys and one girl died in infancy. Six girls (Te Rina, Pareārau, Tai, Hinehou, Hana and Matehuatahi) and five boys (Mākarini Tānara, Paratene Purewa, Wiremu Tuakana, Tīpene and Hēnare) survived into adulthood. In 1899 Apirana was appointed travelling secretary of Te Aute College Students' Association and he and Arihia returned to live in the Ngata family home at Kākāriki near Ruatōria. After Apirana's election to Parliament in 1905 as the member for Eastern Māori, he was often in Wellington. By 1914 he and Arihia had a new house built for their family at Waiomatatini. The house was named Te Wharehou and was referred to as such by the local community, but in later years it came to be more widely known as The Bungalow. Arihia became the sheet anchor of the most important household on the East Coast during her husband's long absences at Parliament and visits to other tribal areas.
Arihia excelled as a homemaker, rearing her children and teaching them about their culture and traditions, gardening, baking bread, making jam, preserving fruit, and welcoming the leading men of Ngāti Porou and many others who came to consult with Apirana Ngata when he was home. She had no servants but relied on help from members of her extended family when catering for large crowds. Arihia Ngata is remembered particularly for her skill at rallying the young people of Ngāti Porou, and her home was a meeting place for them. The tennis courts were an attraction for young players, and Arihia gave her support to this and other sports, including rugby football, basketball and hockey. Visitors, young and old, spent evenings singing around the piano in the 'Māori room', which contained carvings and tukutuku panels celebrating Apirana Ngata's achievements in the Māori and Pākehā worlds.
During the First World War Ngāti Porou leaders encouraged their young tribesmen to enlist in the Māori contingents. Arihia played an important role, organising fund-raising events and providing hospitality to the young recruits. She was a central figure at hui in support of Apirana Ngata's early economic development schemes, and also at the large gathering at Waiomatatini in February 1917 when the decorative carvings and tukutuku panels in Te Wharehou were unveiled. Those present contributed over £3,000 to help with expenses, but Ngata persuaded the donors to put the money towards the establishment of the Māori Soldiers' Fund. In 1918 she was made an MBE for her work during the war. In the post-war period, Arihia provided lodging for young men from other tribes who came to learn sheepfarming skills from Ngāti Porou. They often stayed for months at a time, pending placement on a sheep farm by Ngata when he returned from Wellington. Arihia embraced them all as part of her wider family.
Although she was not fluent in English, Arihia Ngata was never ill at ease in Pākehā company and accompanied Apirana at public functions with grace and confidence. She was described as 'a tiny woman, shy and very gentle', and her tactful and sympathetic presence was a foil to her husband's more assertive approach. She supported all his political activities and shared his commitment to temperance and the Anglican church. When Ngata became native minister in 1928, he depended even more on Arihia (by now Lady Ngata) to stand in for him at Ngāti Porou hui while he was away. For this she was held in high esteem by her people.
In March 1929 Apirana Ngata played a prominent role in the opening of the Tainui meeting house Māhinārangi, at Tūrangawaewae marae in Ngāruawāhia. Arihia and their eldest son, Mākarini, accompanied him, along with a Ngāti Porou contingent of a thousand, to this historic hui which marked the beginning of the cultural renaissance of the Māori. As a mark of appreciation for Ngāti Porou support in raising money for the project, its instigator, Te Puea, gave Arihia the honour of unveiling the carved figure of Pōtatau Te Wherowhero on the front pillar of the house. The difficulty encountered in the unveiling, combined with murmurs of disapproval from some Waikato elders, were subsequently interpreted as portents of the disaster that followed.
Mākarini contracted dysentery at the hui. At home Arihia nursed her son, but he died on 8 April. She too became infected and succumbed 10 days later on 18 April. Arihia's death, combined with rumours of makutu, devastated Apirana and strained his relationship with Te Puea, though he knew Arihia had contracted the disease from Mākarini.
Arihia Ngata's loss was felt by all of Ngāti Porou. The Lady Arihia trophy for Māori women's hockey was established in her memory and the Arihia Memorial Hall was built at Waiomatatini in 1930 to complement Porourangi, the ancestral house of Ngāti Porou. In 1938 the hall was demolished by a flood. It was rebuilt and reopened by Apirana in 1950, shortly before his death. He described this as the 'last job' of his life, a symbolic gesture of gratitude and affection for his beloved wife and the mother of his children. Capable, loyal and hardworking, Arihia Ngata more than met the challenge of supporting her husband's many endeavours, and in doing so earned not only his devotion but the love and respect of all her people.