Siejka was elected as a first-time Labor party candidate to the Tasmanian Legislative Council in a by-election for the electoral division of Pembroke on 4 November 2017.

[2] In her inaugural speech Siejka spoke about the importance of consultation with community members, key issues raised during the election.

[4] In 2020, Siejka was the first woman in the Tasmanian Legislative Council to give birth whilst in office, and to have her daughter in the Chamber with her.

[11] Siejka has served on Boards of Governance across a number of fields, including terms on the Australian Youth Affairs Coalition, National Youth Coalition for Housing, Homelessness Australia,[12] and the Tasmanian Building and Construction Industry Training Board.

[13] In 2016, Siejka was appointed to the board of TasTAFE, as she put it, to "change the culture" of the organisation after an audit by the Integrity Commission revealed incidents of nepotism and credit card misuse.