Activist organisation GetUp spent $10.6 million seeking to influence politicians and the public over the past year.
The spending was revealed in the latest Australian Electoral Commission returns for 2014/15 published on Monday.
The organisation's largest donors over the year included Amanda Lopez from Berry in NSW ($103,125) and New York-based online activist network Avaaz ($99,985).
It was by far the biggest non-party spender in the Australian political scene over the past year.
GetUp's recent campaigns have included same-sex marriage, banning live animal exports, saving Tasmania's forests and stopping coal seam gas.
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Lobby groups, unions and other bodies pumped $14 million into political expenditure, including television campaigns and polling, over the year.
The second-largest amount of political spending was $2.16 million by the peak union body the ACTU.
The World Wide Fund for Nature spent $433,000 on its campaigning for action on climate change and protecting endangered animals, while education unions spent $370,000 seeking a better funding deal for schools.
