Te Whatanui, sometimes known as Te Whata, Toheāpare, or Toheata, was the son of Tihao of Ngāti Huia and Ngāti Parewahawaha, two hapū of Ngāti Raukawa. His mother was Pareraukawa, elder sister of Hape or Hape-ki-tūārangi of Ngāti Raukawa and Ngāti Huia. Hape died at Maungatautari, the heartland of Ngāti Raukawa ancestral territory, which stretched eastwards from Maungatautari towards the Pātetere plateau, about the turn of the nineteenth century, when Te Whatanui was still young. Hape's own sons were also young, and Te Whatanui and the other hereditary chiefs of Ngāti Raukawa made no protest when Te Rauparaha, a contemporary of Te Whatanui and belonging to Ngāti Huia through his mother, claimed the succession to Hape's mana, and later married his widow. Effective leadership devolved on Te Whatanui and other chiefs; Te Whatanui was to be regarded as the leading chief of Ngāti Raukawa.
Ngati Raukawa were increasingly threatened as the wars between Waikato and the Kāwhia tribes continued in the early nineteenth century. After Te Rauparaha had led the first migration of his people to Taranaki about 1821, he returned to attempt to recruit allies to join him in the resettlement of his people on the Kapiti coast. He turned to Ngāti Raukawa first, but Te Whatanui and other chiefs refused his invitation. However, when Te Rauparaha carried the same invitation to Te Waru of Ngāi Te Rangi, at Tauranga, to other chiefs, and finally to Ngāti Whakaue, at Rotorua, Te Whatanui went with him. There, perhaps late in 1821, Te Whatanui urged Ngāti Whakaue and Tūhourangi to kill any Ngāpuhi they might meet, because Whetūroa, his nephew, had been killed that year by Ngāpuhi, at Te Tōtara, in the Thames district. His wishes were heeded, and the killing of many Ngāpuhi by Ngāti Whakaue and Tūhourangi led to the mass Ngāpuhi attack on Mokoia in 1823.
Although he did not accept Te Rauparaha's invitation to resettle in the Cook Strait region, Te Whatanui was determined to establish a new home for his people. His preference was for Heretaunga (Hawke's Bay). On the way there he took a large party including women and children to Taupō. Almost at once his fighting force joined that of Mananui Te Heuheu Tūkino II in taking the pā Te Roto-a-Tara, near present day Te Aute College. While Ngāti Raukawa were settled at Taupō, the chief Te Wharerangi was killed in the Rotoaira district by Te Āti Haunui-a-Pāpārangi of upper Whanganui. Te Whatanui then invaded Whanganui, killing Hekawai. Te Āti Haunui-a-Pāpārangi retaliated, inflicting a heavy defeat on Ngāti Raukawa, and killing Te Ruamaioro. To avenge this death Te Whatanui campaigned against Ngāti Te Ūpokoiri and Ngāti Hinemanu in the upper Rangitīkei district, defeating them at Te Ōtaparoto.
After the victory at Te Roto-a-Tara, Ngāti Te Whatuiāpiti had retreated with Ngāpuhi to Nukutaurua on the Māhia peninsula and much of Heretaunga was deserted. Te Whatanui made his first serious attempt to establish a new home there for his people. He had been invited to come by Te Kaihou, the sister of Te Ringanohu of Ngāti Te Kikiri, a hapū of Ngāti Te Whatuiāpiti, to assist her people against Ngāti Te Ūpokoiri. Te Whatanui brought a party of 150 to 200 fighting men. Those chiefs of Heretaunga, mostly Ngāti Kahungunu, who had not retreated to Nukutaurua, fortified themselves at the Ahuriri pā, Te Pakake. They were prepared to tolerate Te Whatanui if he would help to defend Te Pakake. But Te Whatanui preferred to establish his own pā at Puketapu, on the Tūtaekurī River. At first Ngāti Raukawa lived in peace with the local people. But tensions developed, especially over resources. Te Whatanui was overheard planning to bring the rest of his people from Taupō.
The chiefs of Heretaunga began to assemble their forces at Te Pakake, sending out messages to all who still lived nearby. Puketapu was attacked from two directions; some of its defenders tried to escape by leaping down a cliff into the river. Te Whatanui was chased by Te Paratene Tipitaha, a younger brother of Tuaha of Waimārama. He caught hold of Te Whatanui's garments; Te Whatanui tore them off and escaped naked, hiding in the water until dark. Some of the leading women of Ngāti Raukawa, including one of Te Whatanui's wives and at least one son, were captured; some later escaped.
In revenge for his defeat at Puketapu, Te Whatanui and his war party joined Mananui Te Heuheu and other chiefs in their attack on Te Wera Hauraki, Te Pareihe, and their people in the pā later known as Kaiuku in the Māhia district. Te Whatanui's ally, Te Momo-a-Irawaru of Ngāti Te Koherā and Ngāti Raukawa, was killed at Kahotea pā near Te Roto-a-Tara.
After his defeats in Hawke's Bay Te Whatanui turned his attention to the Kapiti Coast. In the later 1820s he brought a large body of Ngāti Raukawa to the Manawatū coast by way of Rangipō, Turakina and Rangitīkei. On their way Ngāti Raukawa surprised and killed small parties of local people, including Te Wharekī of Ngāti Apa.
Te Whatanui and his party were welcomed by Te Rauparaha, and may have lived on Kapiti Island for some years. Later Te Rauparaha, because of his relationship to Te Whatanui, made him a gift of Manawatū, the Horowhenua district and Ōtaki. He is said to have remarked: 'Live at Ōtaki, but be careful of Muaūpoko'. (Muaūpoko were one of the peoples already living in that district.) Te Whatanui is said to have replied: 'You meddled with them; I shall not…I shall live in peace.' However, Ngāti Apa were planning to attack Ngāti Raukawa because of the deaths they had caused on their migration. While Te Whatanui's party advanced along the beach to Ōhau, Ngāti Apa were trailing them inland. They had left their women and children at the Horowhenua pā, Hotuiti; Te Whatanui got there first, and captured at least twelve Ngāti Apa women. He set one free to bear a message of peace to her chiefs. Ngāti Apa remained suspicious; they attempted to capture a nephew of Te Whatanui; this incident put Ngāti Raukawa on their guard. Te Whatanui brought his force downriver by canoe, his people firing their weapons at both sides of the river. Te Whatanui called out to Ngāti Apa that he wanted to make peace; he assured them that there would be no treachery: 'There will be no evil done, Ngāti Apa'. More captives were released as a sign of good faith.
Te Whatanui succeeded in making peace with Rangitāne, Ngāti Apa and Muaūpoko. He made a famous speech to the 100 or so Muaūpoko still living at Horowhenua, survivors of clashes with Ngāti Toa, offering to be the rata tree that sheltered them. He allowed Ngāti Apa to share Ngāti Raukawa's territory with no loss of mana; later his generosity was to cost Ngāti Raukawa dearly when claims came before the Native Land Court. He settled at the pā Rau-matangi at Horowhenua. After his people were established, Te Whatanui led a war party through the Manawatū Gorge to avenge his defeats in Hawke's Bay. Aided by Ngāti Pakapaka and Ngāti Mutuahi, two hapū of Rangitāne, he raided as far north as Tangoio. Two high-ranked women, Paeroa and Kutia of Heretaunga, were killed by this war party. In response, a combined Hawke's Bay, Wairarapa and Ngāpuhi force attacked Rangitāne living in and near the Manawatū Gorge. About 1829 Rangitāne, Muaūpoko, Ngāti Apa and Ngāti Te Ūpokoiri were invited to a feast by Te Pūoho-o-te-rangi of Ngāti Tama. Te Whatanui was true to his word and warned them not to go. They ignored his advice, and were massacred by Te Āti Awa.
After the arrival of Ngāti Raukawa tensions developed between the migrant tribes themselves. A fatal argument over potatoes stolen at Waitohu by Te Āti Awa triggered the important battle between them and Ngāti Raukawa known as Haowhenua, about 1834. Mananui Te Heuheu of Taupō and other northern chiefs came to the aid of Te Whatanui. His kindness to the local people brought its reward; Ngāti Raukawa were assisted by Rangitāne, Muaūpoko and Ngāti Apa. Nevertheless, Te Whatanui's people did not achieve total victory; both sides in the quarrel were considered winners and losers.
After this battle part of Ngāti Raukawa occupied the Rangitīkei district. Others retained Haowhenua and the Ōtaki district and Te Whatanui continued to live at Horowhenua. Tension eased, only to flare up again in 1839 at the battle known as Te Kūititanga. Ngāti Raukawa attacked Te Āti Awa at Waikanae but were driven back.
There are few glimpses of Te Whatanui after Pākehā settlement began. He extended a welcome to missionaries, but used Henry Williams to carry messages to Mananui Te Heuheu, inviting him to attack Te Āti Awa. He sold land in Manawatū to the New Zealand Company, an action opposed by Te Rangihaeata. He spent much time laying down boundaries as Pākehā interest in the area increased. He was converted to Christianity by Octavius Hadfield, but remained convinced of the powers of tapu and mākutu. By now he was an old man, and younger chiefs had moved to the fore. He may have spent his final years with relatives at Muhunoa, on the Ōhau River. He died early in 1846. He was survived by his sons, Te Tūtaki and Te Tahuri; his daughter, Rangingangana, had been married to Pōmare II in peacemaking some years before.