After her career in state politics, she was an adviser to Senator Bob Brown from 2000 until she was elected to represent Tasmania in the Federal Senate at the 2004 federal election. Preferences to Family First from the Australian Labor Party almost prevented her from being elected; however, she managed to reach a quota mostly as a result of the high level of below-the-line voting in Tasmania. The other Green elected at that election was Rachel Siewert from Western Australia. Milne was part of Bob Brown's frontbench covering the portfolios of Arts, Climate change, Competition Policy & Small Business, Finance & Administration, Food Security, Regional Australia, Resources & Energy, and Trade.