Wellington City Council

Day was the president of the local playcentre and managed the Bikes in Schools programme, where she often lobbied her local councillor Justin Lester, who encouraged her to run for the council herself.^[1]

Day felt urged to run for the Wellington City Council after meeting a homeless family in 2016. Though running as an independent, she stated that "it's pretty clear I'm a left-leaning political being".^[1] In the 2016 elections, she succeeded Justin Lester, who was elected mayor, in the Northern ward, and became the first Māori woman to be elected to the council.^[3] On 28 September 2017, less than a year into her first term, she was appointed deputy mayor by Lester, replacing Paul Eagle whom had been elected to parliament in the general election.^[4] She held this position until 2019 when Lester lost the mayoral election to Andy Foster.

In the 2019–2022 term, she was chair of the Strategy and Policy Committee until June 2021 and thereafter chair of the Social, Cultural and Economic Committee.^[5] In these roles she drove the process to create a Māori ward in Wellington.^[6]

In February 2022, she announced she would not stand again for Council at the 2022 election, instead running for and being elected to the Tawa Community Board.^[7][8] She was elected as an independent candidate, topping the poll with more votes than any other candidate.^[9]

Labour Party

Day joined the Labour Party before the 2020 general election.^[10] In July 2022, she was elected unopposed to be the next president of the Labour Party after Claire Szabó stated she would not seek re-election that year. She was the only nomination to take over from Szabó at the 2022 Labour Party annual conference.^[11][12] Her election as president was confirmed in November 2022 at the conference.^[13]