Labour candidate, 2005–2008

In the 2005 parliamentary election Sio was ranked 47th on the Labour party list and failed to be elected by two places.^[9] However Labour Party list MP Dianne Yates left the Parliament on 29 March 2008,^[10] and Sio was declared elected in her place (the person above him on the list, Louisa Wall, had already been declared elected to replace Ann Hartley).^[11][12]

Prior to entering Parliament, Sio was a representative on Labour's national council as Pacific Islands Vice-President.^[13]

In opposition, 2008–2017

In the 2008 general election Sio won the Māngere electorate, defeating the incumbent independent (and former Labour) MP Taito Phillip Field by 7,126 votes.^[14] In the 2011 and 2014 elections, Sio's majority was approximately 15,000 votes.^[15][16]

In 2013, Sio voted against the Marriage Amendment Bill, which aimed to permit same sex marriage in New Zealand, with fellow Labour MPs Rino Tirikatene, Ross Robertson and Damien O'Connor, alongside New Zealand First, Brendan Horan (former New Zealand First MP), and 32 National MPs.^[17] The Bill passed, becoming law.

In government, 2017–2023

Sio was re-elected during the 2017 general election by a margin of 14,597 votes, defeating National Party candidate Agnes Loheni.^[18] Following the formation of Labour-led coalition government with New Zealand First and the Greens, Sio was appointed as a Minister outside Cabinet by the Labour Party caucus.^[19] On 26 October 2017, Sio was appointed Minister for Pacific Peoples, Associate Minister for Courts, and Associate Minister of Justice.^[20]

During the 2020 general election, Sio was re-elected in Māngere by a margin of 19,396 votes, defeating National's candidate Loheni.^[21]

In early November, Sio became Minister for Courts, while retaining his Pacific Peoples ministerial portfolio. He also retained his associate justice portfolio while picking up the associate foreign affairs, education (Pacific Peoples), and health (Pacific Peoples) portfolios.^[22]

After Prime Minister Jacinda Ardern confirmed that the New Zealand Government would apologise for the Dawn Raids of the 1970s and early 1980s, Sio gave an emotional testimony of his family's experiences with a dawn raid, stating that the apology restored mana for the victims of these raids.^[7][8]

In mid-December 2022, Sio announced that he would not be contesting the 2023 New Zealand general election and would step down at the end of the 2020–2023 term. He attributed his resignation decision to his family and matai (chiefly) responsibilities, stating that his job as a Member of Parliament was "all-consuming and relentless."^[23][24] On 10 February 2023, Sio was granted retention of the title "The Honourable" for life, in recognition of his term as a member of the Executive Council.^[25]