SYDNEY - Australian Senator Christine Milne unexpectedly resigned as leader of the small but influential Greens Party on Wednesday, raising questions over the future of an important political bloc outside the two mainstream parties.
Milne, who took over the party from founder Bob Brown in 2012, quickly built on its early successes by aggressively peeling voters from the main opposition Labor Party's left flank on issues such as climate change and asylum seekers.
Her resignation will leave the party without a proven leader less than two years from general elections. That vote will likely be seen at least partly as a referendum on the Greens' long-term viability as more than just a party of protest.
"It is with a mix of optimism, pride, excitement and sadness that I am resigning the leadership and leaving the Senate," Milne said in a statement.
"My decision to resign today is one I made with my family," said Milne, who is from the island state of Tasmania.
The party is expected to announce a new leader later on Wednesday after a closed-door ballot, the statement said. None of the frontrunners in that ballot can equal Milne's experience.
Milne has been a mainstay of left-wing politics since she was elected to the Tasmanian state parliament in 1989. She graduated to the federal Senate in 2004, where she served as Brown's deputy from 2008 until his resignation.
Despite early scepticism over her ability to replace the charismatic Brown, she led the party from strength to strength in national and state elections.
Her resignation came just weeks after state elections in New South Wales in which the Greens scored their best-ever performance, building on anger among farmers over coal-seam gas mining to capture two traditionally right-wing seats.
Milne has been one of the nation's most vocal critics of conservative Prime Minister Tony Abbott. Her replacement will have the difficult first task of rebutting the government's annual budget next week.