Sage was a field officer and spokesperson for Forest and Bird before being elected as councillor for the Selwyn-Banks Peninsula Regional Constituency of Environment Canterbury at the 2007 elections. She lost her seat when the Environment Canterbury Council were replaced by Commissioners on 1 May 2010.
Sage contested the Selwyn electorate at the 2011 general election for the Green Party. Although she did not win the electorate, she was ranked at sixth on the party's list. The Green Party received sufficient votes to return 14 list members and Sage was elected to Parliament. In addition to her caucus responsibilities as her party's spokesperson on Christchurch, conservation, environment, land information, local government, resource management issues, and water, Sage serves as deputy chairperson of the local government and environment committee.