Taonui Hīkaka was born probably at Paripari (near present day Te Kūiti) in the early 1840s. He could trace his descent from Rakataura of the Tainui canoe. His father was Taonui Hīkaka (also known as Hīkaka) of Ngāti Rora, a hapū of Ngāti Maniapoto. His mother may have been Niapo of Ngāti Hia, who, along with Taonui senior, was part of a group portrait painted by George French Angas in 1844. Taonui is believed to have had four brothers, Te Kurī (Ngāturi), Tanirau, Te Naunau Hīkaka and Tautau; and a sister, Te Rangituatahi, who married the trader Louis Hetet.
Taonui's father played a central role in the wars between Waikato and Taranaki tribes in the 1820s and 1830s. He took the name Hīkaka (angry) for his pursuit and killing of the warriors Te Tupeotū and Te Hautehoro after the siege of Rangiuru pā near the mouth of the Ōtaki River in 1834. His mana was so great that he was considered a candidate for the leadership of the King movement. He is said to have been the only man who dared to challenge the insult to Ngāti Maniapoto in a song written by Pōtatau Te Wherowhero. He demanded utu. Pōtatau responded by giving him the suit of armour which King George IV had given to Hongi Hika in England, and which Hongi had given to Pōtatau.
The young Taonui probably accompanied his father and an ally, Tāwhana Tīkaokao, in a group of Ngāti Maniapoto who went to Waitara in April 1863 to attack government troops there. Fighting alongside his father would have been a fiery apprenticeship for the young Taonui. He emerged from the war with great mana.
Taonui's father is thought to have died in the 1860s. From this time Taonui assumed the leadership of Ngāti Rora, and with Wahanui Huatare and Rewi Maniapoto formed a strong triumvirate as the leading Ngāti Maniapoto chiefs, although Taonui was considerably younger than his colleagues.
From the early 1870s the problem of confiscated Waikato land made the wartime alliance of Waikato and Ngāti Maniapoto increasingly fragile, and caused tensions among Ngāti Maniapoto. In 1875 Donald McLean, the native minister, met with Tāwhiao and offered some local independence to the King movement, but stated that the return of all the confiscated lands was impossible. A similar offer was made by Premier Sir George Grey in January 1878, during a meeting at Te Kōpua at which Rewi was present. Both offers were rejected.
Taonui was present when Grey met Rewi at Waitara in June 1878. Rewi and Taonui remained at Waitara until late in the year, ignoring a request from Tāwhiao's council and Ngāti Maniapoto to return to Waikato. Many Ngāti Maniapoto felt that they were acting without authority, and feared that they were making conciliatory approaches to the government. After their eventual return, Taonui defended Rewi's stay in Waitara to a Ngāti Maniapoto meeting in January 1879 at Te Kōpua.
By 1880 problems connected with land sales began to prove increasingly troublesome for Ngāti Maniapoto, still nominally party to the King movement's ban on land sales and land court operations. In 1880 Tāwhiao tried to base the King movement at Hikurangi, south of Pirongia Mountain, in an attempt to stop its adherents from dealing in land; Rewi, Taonui and Wahanui consented to the move to Hikurangi. In 1883 they sent a lengthy petition with 412 signatures to the governor and both houses of Parliament, complaining bitterly of legislation that deprived them of 'the exclusive and undisturbed possession' of their lands as promised in the Treaty of Waitangi.
Claims by other tribes to Ngāti Maniapoto lands and fierce government pressure led growing numbers of that tribe to challenge those among their chiefs, including Wahanui and Taonui, who remained loyal to the King movement's land policy. The Native Land Court was increasingly asked to define the boundaries for Ngāti Maniapoto lands. In November 1883, at a meeting with Minister for Native Affairs John Bryce at Kihikihi, tribal leaders laid claim to almost 2.4 million acres from Waikato as far south as Ruapehu. While at Kihikihi, Taonui co-operated with Bryce's inquiry into the 1880 killing of a European who had trespassed into the King Country.
Eventually Taonui and Wahanui were forced to abandon the isolationist policy of the King movement. On 1 December 1883 they and Rewi formally agreed to a survey of the external boundaries of the King Country by S. P. Smith. Taonui, afraid of losing control of land dealings, was insistent that the survey should not lead to immediate subdivision. Having made the irreversible decision to allow surveys, Taonui and Wahanui co-operated with the government by helping to remove obstructions to them. Taonui strongly backed the building of the main trunk railway line, and wrote to the native minister, John Ballance, at the end of 1884 to express his support. Wahanui, Rewi and Taonui were involved in turning the first sod on the line on the bank of the Pūniu River on 15 April 1885. Earlier that day Taonui had declared in front of Premier Robert Stout, 'It is our land; the sod shall be turned, and turned today!' after Stout had been told by two Waikato chiefs opposed to the railway that it was Waikato land.
Taonui, however, was unhappy with the Native Land Court and the lack of power given to the Native Affairs Committee in Parliament. He believed that there should be more Māori seats in Parliament and that land should be vested in hapū and controlled by hapū committees, not by individuals. On this he had a sympathetic ear from Ballance and they met a number of times in the following years. In 1886 Ballance introduced Taonui to the governor in Auckland.
From 1886 Taonui became embroiled in a dispute with Ngāti Tūwharetoa over the Taupōnui-a-Tia block near Lake Taupō. That year he missed a court hearing on the case, and arrived to hear that the court had awarded the land to Ngāti Tūwharetoa. He had requested a postponement, and now asked for a rehearing and time to report to his tribe; when the judge took no notice he threatened to leave and was arrested for contempt of court. The dispute was brought up in Parliament, and eventually settled by a commission of inquiry. Also in 1888 he petitioned the Native Affairs Committee asking that the chief judge of the Native Land Court be replaced by a person acquainted with Māori language and customs, and that changes be made to the way the court was run.
In 1891 Taonui appeared before the Native Land Laws Commission, asking for the removal of restrictions on leasing Ngāti Maniapoto land. He referred again to the Taupōnui-a-Tia dispute, asked that more time be allowed for Māori to pay survey costs, and said that the number of Māori MHRs should be increased. Taonui and Wahanui met with the native minister, Alfred Cadman, in December 1891; they still firmly opposed unrestricted sales of land, but again asked to be allowed to lease to private individuals. They also asked that Cadman use his power to make land laws less obnoxious to Māori.
Taonui, like Wahanui, was of massive size – over six feet tall. He is depicted in a portrait by Gottfried Lindauer, although it was probably painted after his death from a photograph. Lindauer seems to have added four lines of moko from the bridge of the nose across the forehead, identical to those George French Angas painted on Taonui's father in 1844; they do not appear in photographs of Taonui.
Taonui was second only to Wahanui in his influence on the King Country in the 1880s. There is little record of him speaking in public. Like Wahanui, he exerted his influence to try to ensure that the opening of the King Country would not be on terms unfavourable to his people. He died suddenly at Te Kūiti on 2 December 1892, and within five days of his death some 800 people had arrived for his tangihanga. He was survived by a wife, whose name is unknown, but had no children. Taonui's land claims were extensive and he was owner or part owner of a large estate at his death. His estate and lands are said to have gone to his brother Te Naunau Hīkaka.