The target matches one set in 2007 by the European Union, which leads the world in green power technology. Many U.S. states also have set renewable energy targets although there is no national goal.
But critics argue the Australian target will make electricity more expensive in coal-rich Australia without curbing the amount of climate-warming carbon gases that the nation emits, as overall electricity consumption rises.
Currently, 8 percent of Australia's electricity comes from renewable sources, including hydroelectric generators built late last century, according to the private Clean Energy Council.
The main opposition Liberal Party on Wednesday promised its support for an amended version of the government-proposed legislation in the Senate, where the ruling Labor Party needs the votes of at least seven opposition senators to pass laws.
The Liberals' deputy leader in the Senate, Eric Abetz, said his party had achieved "about 80 percent of what we wanted" in changes to the government's plan.
The amendments increase government assistance to industries that are heavy users of electricity and create safeguards for existing investment in the coal mining industry.
Junior Climate Change Minister Greg Combet said the Liberals' decision to support the bill in a vote either Wednesday or Thursday is "a welcome development which is respected by the government."
But Climate Change Minister Penny Wong told the Senate that even with one fifth of Australia's electricity coming from renewable sources by 2020, the nation's carbon gas emissions are projected to be 20 percent higher than 2000 levels.
"The only way we're going to be able to turn around the growth in our carbon pollution ... is to put a firm legislated limit on the amount of carbon that we produce and make those who create the pollution pay for it," Wong said.
Last week the Senate rejected a government-proposed bill that would have taxed industries' carbon emissions starting in 2011 and slashed the country's emissions by up to 25 percent below 2000 levels by 2020.