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Bob Brown resigned Friday as leader of Australia's increasingly influential Greens party and will leave Parliament in June after 16 years.

The surprise resignation won't alter a deal agreed in 2010 allowing the center-left administration of Prime Minister Julia Gillard to form government with a one-seat majority, Brown said. His resignation also will not force a by-election as his seat will automatically be filled by a Greens party member.

But it does leave a big void in left-of-center politics, where Brown, 67, is widely credited with steering the Greens from a minority environmentalist party a decade ago to an emergent third force in politics, winning a record 12% of the vote in the 2010 federal election.

The party has nine seats and holds the balance of power in the country's upper house Senate, up from just one seat a decade ago. It also won a seat in the lower House of Representatives in 2010.

"The Greens have such a depth of talent and experience lined up for leadership--I could only dream about that a decade ago," Brown said Friday. "It is prime time to hand over the reins."

Christine Milne, 58, a fellow Senator for the island state of Tasmania, will take over the leadership.

Milne has been a vocal advocate for policies like a tax on greenhouse-gas emissions and the profits of mining companies that convinced the Greens to back Gillard's Labor government in 2010, over the conservative Liberal-National coalition led by Tony Abbott, after a hung ballot left neither of Australia's two major political forces with enough votes to govern alone.

"The mining boom will end and climate change is coming. And with the big end of town more aggressive than we've seen for decades in demanding weaker environmental protection and weaker industrial relations laws, our country faces a stark choice between two futures," Milne said.

"Will we choose to put profits ahead of people and planet, digging ourselves deeper into a hole? Or will we prepare our country for the enormous challenges of the 21st century while building a better quality of life and a fair go for all of us?," she said.

Australia's mining industry earlier Friday launched a pre-emptive strike against the threat of further tax increases, resurrecting the "keep mining strong" slogan used two years earlier to thwart a proposed super profits tax.

The Minerals Council of Australia began running full-page advertisements in newspapers cautioning against possible new or increased levies in the Labor government's next budget, due May 8. The ads are set to run in major papers over the coming days into next week.

The council--which represents major minerals producers including BHP Billiton Ltd. (BHP), Rio Tinto PLC (RIO) and Xstrata PLC (XTA.LN)--had agreed an advertising truce with Gillard in mid-2010 after its intensive campaign against the planned super tax rattled support for the government and led to the ouster of then Prime Minister Kevin Rudd. Gillard went on to water down the mining tax and narrowed its focus to coal and iron ore after consulting with the industry.

-By Rachel Pannett, Dow Jones Newswires; 61-2-8272-4684; rachel.pannett@dowjones.com

--Robb M. Stewart in Melbourne contributed to this article

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