Bob Brown's leadership (2001–2012)
In the 2001 federal election, Brown was re-elected as a senator for Tasmania, and a second Greens senator, Kerry Nettle, was elected in New South Wales. The Greens opposed the Howard government's Pacific Solution of offshore processing for asylum seekers, and opposed the bipartisan offers of support to ANZUS and the Afghanistan War by the government and Beazley Opposition in the aftermath of the September 11 attacks. The party described the Afghanistan commitment in particular as "warmongering". Brown and Nettle's performance in the Senate increased voter support for the party, as it showcased that the Greens were not deep ecologists nor a single-issue environmentalist organisation, thus granting approval from disaffected Labor voters.
On 19 October 2002, the Greens won a House of Representatives seat for the first time when Michael Organ won the Cunningham by-election, triggered by the resignation of Labor member Stephen Martin.
In the 2004 federal election, the Australian Greens fielded candidates in every House of Representatives seat in Australia. The Greens' primary vote rose by 2.3% to 7.2%. This won them two additional Senate seats, taken by Christine Milne in Tasmania and Rachel Siewert in Western Australia, bringing the total to four. In New South Wales, the Greens ran John Kaye as their lead Senate candidate but were unsuccessful due to unfavourable preference flows, and Organ lost his seat to Labor's Sharon Bird.
At the 2004 election, the Australian Greens replaced the Australian Democrats as the largest third party in elections, a title that they have held since.
The Greens increased their national vote by 1.38 points to 9.04% at the 2007 federal election, with the election of South Australian senator Sarah Hanson-Young taking the number of Greens senators to five. Senators Bob Brown (Tas) and Kerry Nettle (NSW) were up for re-election, Brown was re-elected, but Nettle was unsuccessful, becoming the only Australian Greens senator to lose their seat despite increasing her vote from 2001.
In November 2008, Senator Christine Milne was elected deputy leader in a ballot contested against Senator Rachel Siewert.
In 2009, the Greens and the Liberal Party voted to defeat Labor's emission trading scheme legislation after failed negotiations for an emissions cut target.
The 2010 federal election marked a high point for the Greens electorally, with the party receiving its largest vote to date and sharing the balance of power. The Greens received a four percent swing to finish with 13 percent of the vote in the Senate. The Greens won a seat in each of the six states at the election, bringing the party to a total of nine senators from July 2011, holding the balance of power in the Senate. The new senators were Lee Rhiannon in New South Wales, Richard Di Natale in Victoria, Larissa Waters in Queensland, Rachel Siewert in Western Australia, Penny Wright in South Australia and Christine Milne in Tasmania. Incumbents Scott Ludlam in Western Australia, Sarah Hanson-Young in South Australia and Bob Brown in Tasmania were not due for re-election. The Greens also won their first House of Representatives seat at a general election, the seat of Melbourne with candidate Adam Bandt, who was a crossbencher in the first hung parliament since the 1940 federal election. Almost two weeks after the election, the Greens agreed to support a Gillard Labor minority government on confidence and supply votes. Labor was returned to government with the additional support of three independent crossbenchers.
Prior to the 2010 federal election, the Electrical Trades Union's Victorian branch donated $325,000 to the Greens' Victorian campaign – the largest political donation ever directed to the Party up to that time.
The Greens signed a formal agreement with the Australian Labor Party involving consultation in relation to policy and support in the House of Representatives in relation to confidence and supply and three of the independents declared their support for Labor on confidence and supply, allowing Gillard and Labor to remain in power with a 76–74 minority government.
On 24 February 2011, in a joint press conference of the "Climate Change Committee" – comprising the Government, Greens and two independent MPs – Prime Minister Gillard announced a plan to legislate for the introduction of a fixed price to be imposed on "carbon pollution" from 1 July 2012 The carbon price would be placed for three to five years before a full emissions trading scheme is implemented, under a blueprint agreed by a multi-party parliamentary committee. Key issues remained to be negotiated between the Government and the cross-benches, including compensation arrangements for households and businesses, the carbon price level, the emissions reduction target and whether or not to include fuel in the price.
In April 2012, Bob stepped down as leader of the Australian Greens, and retired from the Senate in June 2012.
