The Australian Greens assume the balance of power in the Senate today, with four new Greens senators who were elected last year starting their terms.

That means there will be nine Greens senators, with newcomers Larissa Waters, Penny Wright, Lee Rhiannon and Richard Di Natale joining their five re-elected colleagues.

The Senate has 76 seats, and as of today it will be comprised of 31 Labor members, 29 Liberals, the nine Greens, five Nationals, one independent and one senator from the Democratic Labor Party.

That means no side of politics can win a majority of 39 votes in the Upper House without the Greens.

Following are details of the Australian Greens senators who will help shape any future decisions in the Senate.

Bob Brown (Tas) - party leader

First elected: 1996

First elected: 1996 Term expires: 2014

Term expires: 2014 Portfolios: Prime Minister and Cabinet; Treasury; Foreign Affairs; Defence and National Security; Schools and Education; Forests; and Whaling and Antarctica

Bob Brown has been a member of the Senate for the past 15 years and became the Greens parliamentary leader in 2006. The 66-year-old former doctor and Tasmanian Wilderness Society director has been recognised as the first openly gay member of Australian Parliament. He has campaigned against deforestation and nuclear energy, while backing same-sex marriage, the rights of asylum seekers and action on climate change.

Christine Milne (Tas) - deputy leader

First elected: 2005

First elected: 2005 Term expires: 2017

Term expires: 2017 Portfolios: Climate Change; Resources and Energy; Trade; Finance and Administration; Competition Policy and Small Business; Arts; and Food Security and Regional Australia

Christine Milne, who was Tasmania's Greens leader from 1993 until 1998, was jailed for three days over the Franklin Dam blockade in 1983 and was known for her staunch opposition to the development of a pulp mill during the late 1980s. Before jumping from state to federal politics, she was an adviser to Senator Brown and was vice-president of the World Conservation Union.

Rachel Siewert (WA) - party whip

First elected: 2005

First elected: 2005 Term expires: 2017

Term expires: 2017 Portfolios: Family, Community and Disability Services; Fisheries and Marine; Ageing; Indigenous Affairs and Indigenous Health; Agriculture; The Kimberley and Northern Australia; and Natural Resource Management

As a former coordinator of the Conservation Council of WA, 49-year-old Rachel Siewert has lobbied against the logging of old-growth forests and pushed for environmental protections and conservation. She was involved in negotiating last year's alcopops legislation, as well as launching calls for an inquiry into the 2009 Montara oil spill.

Sarah Hanson-Young (SA) - party room chair

First elected: 2008

First elected: 2008 Term expires: 2014

Term expires: 2014 Portfolios: Immigration and Citizenship; Consumer Affairs; Water and the Murray-Darling Basin; Human Rights; GLBTI; Youth; Childhood Education and Care; and Tibet

At 29, Senator Hanson-Young was one of the youngest people ever elected to the Senate and the second youngest woman ever elected to Parliament. She previously worked as a campaign manager for Amnesty International and media adviser to Greens politician Mark Parnell.

Scott Ludlam (WA)

First elected: 2008

First elected: 2008 Term expires: 2014

Term expires: 2014 Portfolios: Broadband; Infrastructure; Communications and Digital Economy; Housing; Nuclear; Mining (WA); Transport; Waste and Sustainable Cities; Burma; and Assisting on Defence

Senator Ludlam, 41, is a former filmmaker, artist and graphic designer who has strongly opposed a radioactive waste dump in a community near the Northern Territory's Tennant Creek.

Larissa Waters (Qld)

First elected: 2011

First elected: 2011 Term expires: 2017

Term expires: 2017 Portfolios: Environment; Biodiversity and Natural Heritage; Population; Great Barrier Reef and Coral Sea; Cape York; World Heritage; Tourism; and Mining

As the Greens' senator-elect for Queensland, former environmental lawyer Larissa Waters ran for the Senate unsuccessfully in 2007 but then won election at the 2010 federal election. Senator Brown has named her as a potential future Greens leader.

Lee Rhiannon (NSW)

First elected: 2011

First elected: 2011 Term expires: 2017

Term expires: 2017 Portfolios: Democracy; Local Government; Higher Education; Assisting on National Security; Women; International Aid and Development; Animal Welfare; and Forests

Lee Rhiannon, 60, joined the Greens in 1990 and was a member of the NSW Upper House from 1999 to 2010. Earlier this year, Senator Brown reprimanded her for advocating a trade boycott against Israel.

Richard di Natale (Vic)

First elected: 2011

First elected: 2011 Term expires: 2017

Term expires: 2017 Portfolios: Health, including preventative; Dental Health; Sport; Gambling; Multiculturalism; East Timor; and West Papua

As a former doctor and public health specialist, Richard Di Natale, 41, has worked in Aboriginal health, HIV prevention and drug and alcohol issues. He ran for the position of Melbourne Lord Mayor in 2004, but was defeated by John So.

Penny Wright (SA)

First elected: 2011

First elected: 2011 Term expires: 2017

Term expires: 2017 Portfolios: Attorney General (excluding National Security); Native Title; Veterans Affairs; Social Inclusion; Mental Health; and Heritage

Penny Wright, 50, is a former solicitor and university lecturer who says she has a particular interest in tenancy law, mental health, social security, refugees and campaigns against violence against women and children.