Siewert was the coordinator of the Conservation Council of Western Australia from 1987 to 2004, initially as the organisation's only paid staff member.[3] In 2003 she was awarded the Bessie Rischbieth Conservation Award.[1]
Siewert was co-convener of The Greens (WA) from 2002 to 2004.[5]
Siewert is the Australian Greens Whip, chairs the Senate Community Affairs References Committee and is a member of the Expert Panel on Constitutional Recognition of Indigenous Australians.[7][8] She served as co-deputy leader of the parliamentary Greens from November 2017 to December 2018, alongside Adam Bandt.[1]
As of 2020, Siewert acts as the Greens' spokeswoman for the portfolios of health; mental health; family, ageing and community services; and gambling.[9]
Siewart has served on the "Inquiry into the destruction of 46,000 year old caves at the Juukan Gorge in the Pilbara region of Western Australia", which delivered its interim report in December 2020.[10]
In July 2020, Siewert announced that she would not recontest her seat at the 2022 federal election.[11] In August 2021, she announced she would resign from the Senate in September 2021, creating a casual vacancy.[12] In September 2021, Siewart formally resigned from the Senate.[13] A joint sitting of the Parliament of Western Australia appointed Dorinda Cox to fill the vacancy on 14 September 2021.[14]