"We will be tackling some complex stuff together on behalf of our city and I'm looking forward to working with you again. Welcome to you and to all those who have come to support you here," Southgate said.
Educated at the University of Waikato, Mr Rau's governance experience includes being an executive member of Te Whakakitenga o Waikato (Te Arataura), a trustee and committee member of Te Kotahitanga Marae and a tribal representative of the Joint Management Committees of Hamilton City Council and Waikato Regional Council.
Mangai Māori, which means 'The voice of Māori', were added to the council's governance structure in 2018 to represent iwi (Waikato-Tainui) and maatawaka (other Maaori who live in Hamilton).
Nominated by Waikato-Tainui and Mana Whenua, Mr Rau joins four fellow Māori representatives who sit on the other council committees.
He replaces Bella Takiari-Brame who resigned from the position earlier this year. She was among the first Mangai Māori appointed by the council.
Mayor Southgate thanked Bella for the work she had done and said council was sorry to be losing her.
"In your own unique way, you have kept us on track on a range of issues and we are grateful for the time you have put in."