Partial results showed the party's share of the vote rose from five percent in 2001 polls to about seven percent, putting it ahead of the centrist Democrats and behind only the victorious Liberal-National coalition of Prime Minister John Howard and the main opposition Labor Party.

Widely spread out across the country, the increase in votes did not prevent the left-leaning Greens losing their only seat in the lower House of Representatives to the Labor Party.

But they looked set to add one or more seats to the two they already hold in the 76-member upper house Senate, where the opposition is expected to maintain its legislation-blocking majority.

In the island state of Tasmania, where a stand against logging of ancient forests lost Labor two key marginal seats due to a backlash from the timber industry, the same issues worked to the Greens' favour, as candidate Christine Milne appeared set to enter the Senate.

There were also suggestions they could pick up extra Senate seats in other states once all the votes were counted.

Green Senator Kerry Nettle said the showing had made the Greens the third force in politics.

"It's about speaking out right from the outset on the war on Iraq, speaking up for Tasmanian forests for decades," said Nettle.

"It's part of a consolidation of a core Green vote that we are seeing across the electorate as the Greens gain a higher profile as the third force in politics."

The Greens and their high-profile leader, Senator Bob Brown, have attracted a profile far above the size of their representation in parliament, largely through outspoken stands on controversial issues.

The party has led campaigns for water conservation after two years of serious drought in most of eastern Australia has left river systems and water tables devastated.

They strongly back the Kyoto Protocol on reducing production of greenhouse gases blamed for global warming and which Howard's government has refused to sign.

They also strongly opposed Howard's decision to join the US-led war in Iraq and have been outspoken in support of two Australians held as terrorist suspects by the United States in Guantanamo Bay.

The Australian Democrats, widely seen as the Greens' chief rivals, said the Greens' policies may have had a detrimental effect on the Labor Party by pushing it to the left and thus alienating voters.

Democrats leader Andrew Bartlett compared it with the role played by Ralph Nader in the United States in 2000, which helped lose the election for Al Gore.

"I almost fear that perhaps Bob Brown's playing a bit of a role like Ralph Nader did in the US of Ralph Nader delivering George Bush the presidency," Bartlett told Sky News.

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