[5] Between secondary school and university, Sage worked as a cleaner and kitchen hand at an Auckland retirement village and rest home.

[2][8][10] Working for Forest & Bird, she gained a public profile advocating against the environmental impacts of logging, mining, and dairy farming.

[13][14] She was aligned with the centre-left bloc of councillors who supported Sir Kerry Burke in the council's leadership elections in October 2007 and September 2009.

[15][16] On the council, Sage was assigned responsibility for pest management and oversaw measures to control weeds, possums, and wild goats.

In her maiden statement delivered on 15 February 2012, Sage said she would not have run for parliament if the Environment Canterbury regional council had not been replaced with commissioners.

[28] She also spoke of her ambitions to create new marine protected areas around the Kermadec Islands and in the Ross Sea, and to enact stronger plant and water conservation laws.

[33] Following the formation of the Sixth Labour Government, Sage assumed the ministerial portfolios for Conservation and Land Information, and Associate Minister for the Environment.

[37] As Conservation Minister, Sage led the development of Te Mana o te Taiao, the Aotearoa New Zealand Biodiversity Strategy,[38] began a governance review of Fish & Game New Zealand,[39][40] and initiated a controversial cull of imported Himalayan tahr on conservation land which resulted in her receiving death threats and was eventually scaled down.

[41][42][43] She established the $1.2 billion Jobs For Nature programme as part of the government's COVID-19 relief efforts and opened the Paparoa Track Great Walk in 2020.

[44] However, she failed to deliver a planned and funded drylands park in the Mackenzie Basin or a proposed prohibition on mining on conservation land.

[31][52][53] She was also appointed the Green Party spokesperson for conservation, emergency management, the environment, forestry, land information, three waters, and oceans and fisheries.

[56][57] Public consultation on the issue was intended to be carried out from September 2018;[58] however, in early 2020 Sage acknowledged that it had not been possible for Labour, the Greens and New Zealand First to agree on a discussion document.

[61][62] In early December 2022, Leader of the House and Labour MP Chris Hipkins announced that the anti-privatisation entrenchment clause would be removed.