Peter made the transition from being a community leader in the campaign to stop the Tamar Valley pulp mill to Greens Senator for Tasmania in 2012 when he replaced Bob Brown in the Senate. He was re-elected at the 2013 election.

Peter's portfolios for the Greens are Agriculture, Healthy Oceans, Science, Industry & Innovation, Trade & Tourism, and Waste & Recycling.

He has campaigned to stop seismic testing and new oil and gas projects in the ocean as well as shark conservation and protection of the Great Southern Reef. As Agriculture spokesperson he has championed the hemp, medicinal cannabis and organic industries. He has also spent the past 20 years working towards building a circular economy, including recently seeking to legislate national producer responsibility schemes.

Peter chaired the Senate Select Committee on Information Integrity on Climate Change and Energy from September 2025 until March 2026. The Committee received 243 submissions and held 11 public hearings. The final report provides a detailed picture of the challenges presented by mis- and disinformation as we transition to renewable energy.