Australian officials plan to build a 140-turbine wind farm in Victoria, the nation’s most densely populated state, by 2013, a billion-dollar project that would be the largest in the Southern hemisphere. The wind farm will provide enough electricity for 220,000 homes and is key to the nation’s new target of meeting 20 percent of its energy needs with renewable sources by 2020. The wind farm, to be built by Australian energy retailer A.G.L. Energy and New Zealand’s state-owned Meridian Energy, would be a significant step for a nation that has yet to tap into its abundant renewable energy resources, including wind, solar, and wave energy. Only about 6 percent of Australia’s energy comes from renewable sources. In June, however, lawmakers passed more ambitious green energy targets, earmarking about 20 billion Australian dollars ($18 billion) for clean energy technologies and a significant expansion of the nation’s electricity grid to connect solar or wind farms in the desert interior with population centers on the coasts. Officials say the proposed wind farm would reduce carbon emissions by about 1.7 million tons annually — the equivalent of taking 420,000 cars off the road.