As it is, unhappiness with Labor and a preferences deal with the Government appear almost certain to increase the number of Greens in the Senate from five to seven, giving the party the balance of power.
That means whoever wins government will have to win its support to pass legislation, handing the Greens clout not seen since the now-defunct Democrats were at their peak. How that power is exercised is what concerns the main players. Some believe the Greens will be driven by a left-wing agenda, others that negotiations will be more considered. Greens Leader Bob Brown says the criteria will be common sense.
The Greens grew as traditional support for the main parties weakened, evolving from the United Tasmania Group set up in 1972 as the world's first Green party to contest plans to flood a mountain lake for hydro-electricity.
But it was the plan to dam Tasmania's Franklin River that really changed Australia's political landscape, thrusting the environment to the centre of federal politics and adding to the dissent that saw the brief rise of the Nuclear Disarmament Party and the greater success of the Democrats, both of which absorbed green influences.
Environmental candidates mushroomed in both federal and state politics through the 1980s, winning seats in the West Australian, Tasmanian and ACT parliaments and gaining two Senate seats in the early 1990s.
But it was the decision to link state bodies into the Australian Greens, and the arrival in the Senate of long-time activist Bob Brown that pushed the party towards real power.
As the Democrats disintegrated the Greens thrived. With Family First Senator Steve Fielding and South Australian independent Nick Xenophon, they hold the balance of power in the 76-seat Senate. The Coalition has 37 senators, Labor 32.
Only half of the Senate faces re-election on August 21, including two Greens - Christine Milne in Tasmania, and Rachel Siewert in WA - both of whom are expected to hold their seats.
New senators will not take their seats for six months, except for those from the ACT and the Northern Territory, who begin work immediately. If Hatfield Dodds wins, the Greens would have six senators from day one, increasing their influence ahead of the arrival of others likely to be elected. As it stands, most analysts expect the Greens to win Fielding's Victorian seat and another in Queensland, giving them seven senators and the balance of power.
Some forecasts predict the Greens could win as many as 10 seats.
On Thursday a Morgan poll said support for Labor in the Senate had fallen to 40 per cent and the Coalition to 36 per cent. The Greens vote had risen 6.5 per cent to 15.5 per cent.
Pollster Gary Morgan said that on these figures the Greens would have 10 senators, Labor 33 or 34 and the Coalition 31 or 32, plus Xenophon.
But Morgan said the poll had overstated the Greens' real polling-day support in previous elections, a trend noted by other analysts as people unhappy with the main parties "park" their votes until confronting ballot papers.
Votes for Lower House MPs are cast in order of preference. If the first choice does not win, the vote passes to the second candidate on the list of preferences, and continues until the final allocation of preferences is determined.
The Greens have reached a deal with Labor, under which the Government will advise its supporters to place the Greens second in their Senate listings, while the Greens will do the same in 54 of Labor's most marginal seats.
The final choice remains with individual voters, but the advice is potent. While helping cement the Greens' grip on the Senate, the allocation of its preferences could be crucial in keeping Labor in office, potentially outweighing the advantage polls indicate the Coalition holds in primary votes. It is the two-party preferred vote, rather than first choices only, which determines government.
Whichever party finally wins power will need to deal with the Greens to push its legislative agenda through the Senate. Brown has said the Greens do not intend to become a blocker, but intense negotiation will become part of the Government's everyday life.
The Greens' support lies mainly in the cities among young, university-educated professionals or public servants, and frequently religious non-believers - a factor that sparked a spat between Brown and Sydney Catholic Archbishop George Pell, who described them as "antagonistic to the faith".
Although rooted in environmentalism, the Greens have a broad policy base, ranging from indigenous affairs, child, family and health issues to industrial relations, human rights, immigration and science. The party supports Labor's new national broadband network, an even higher rate of tax on miners, the use of higher fees to force a shift from road to rail transport, and a high-speed rail network down the east coast.
And the Greens remain strong advocates of carbon taxes.
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Labor - 51 per cent
Coalition - 49 per cent
Western Australia:
Coalition - 58 per cent
Labor - 42 per cent
Queensland:
Coalition - 54 per cent
Labor - 46 per cent
Victoria:
Labor - 55 per cent
Coalition - 45 per cent
South Australia:
Labor - 56 per cent
Coalition - 44 per cent
New South Wales:
Labor - 51 per cent
Coalition - 49 per cent
