On 3 December 1846, Brunner began what became his longest and most arduous expedition. He planned to follow the Buller River to the sea and then trek down the West Coast as far south as Milford Sound. During his previous expedition, he had been told of the existence of a route through the Southern Alps by the Māori at the Arahura River. He hoped to discover this route and use it to cross the Southern Alps and reach Canterbury. He was accompanied once again by Kehu, who brought along his wife. Another Māori, Pitewate, a friend of Kehu's, also joined the venture, accompanied by his wife. Brunner provided clothing and shoes for his companions. The wives proved problematic during the journey as they quarrelled, sometimes supported by their husbands, and Brunner would have to mediate.