SYDNEY--Falling commodity prices have opened up a US$20 billion hole in Australia's export revenues, intensifying gloom in the mining sector as companies scramble to restructure debt and rethink billions of dollars of new investments.

Australia's government Tuesday scrapped expectations that it will post another year of record earnings from exporting commodities such as coal and iron ore because of softening prices and warned that up to 230 billion Australian dollars (US$241 billion) of new investments are at risk unless the industry can bring costs down and improve productivity.

In a sign of the strain on the resources sector, Fortescue Metals Group (FMG.AU)--the world's fourth-largest iron ore producer by volume--said it has secured US$4.5 billion in debt to refinance loans and stave off a potential fire sale of assets after being caught out by the sudden fall in the iron ore price this month.

Australia has relied on the mining industry to shield it from the economic headwind buffeting Europe and the U.S., where high unemployment and fears over sovereign debt continue to smother their recovery. China's spending boom on infrastructure following the financial crisis in 2009 fueled sharp growth in Australian resources companies' profits, underpinned high commodity prices and spurred investment in projects down under ranging from gas-export terminals to vast new iron-ore pits.

But pressure on mining companies is growing as Asia's economic outlook dims. Economic growth in China--Australia's biggest trading partner and a key buyer of its energy and raw materials--has slipped to the slowest rate since 2009 and exports from labor-intensive manufacturing hubs Zhejiang and Jiangsu provinces are falling.

That has driven down prices of industrial commodities such as thermal coal, used to generate electricity, to levels of half their peak in mid-2008 while iron ore hit a three-year low earlier this month. Iron ore and coal are Australia's two biggest exports, respectively.

New Hope Corp. (NHC.AU), one of Australia's largest coal miners, said Tuesday that it could delay plans to expand a coal mine and build new ones in Queensland state by up to a year and hasn't ruled out job cuts as margins come under pressure from the high Australian dollar, new taxes and weak prices. It followed recent moves by heavyweight peers including Xstrata PLC (XTA.LN) and BHP Billiton Ltd. (BHP) to cut costs, including by laying off workers.

"Although this is not good news, it is by no means a death knell for the Australian resources industry," Australia's resources minister Martin Ferguson told a conference in Canberra.

The Bureau of Resources and Energy Economics, a key government forecaster, said Tuesday that it expects export earnings from minerals and energy for this financial year ending June 30, 2013 to fall 2% to A$189 billion because of tumbling commodity prices. The latest estimate represents a 10% decline from its initial A$209 billion forecast issued just three months earlier.

The stakes are high as Australia's minority center-left Labor government is depending on mining taxes and royalties to return the national budget to a slim surplus by mid-2013 ahead of general elections due later that year. In May, it announced a A$33 billion public spending savings package, its biggest in decades, in a bid to achieve its increasingly fragile budget surplus target.

Mr. Ferguson said even though the current cycle of high commodity prices is over, the resources sector in Australia will continue to prosper, as more than A$270 billion of investment has already been locked in--mostly for the construction of terminals to supply Asia with liquefied natural gas.

But a question now is whether Australia can capitalize on the next wave of demand growth for iron ore used to make the steel in Asia's high-rise buildings and coal, and the government is working with the resources industry to upgrade infrastructure, expand the pool of skilled workers and improve productivity, Mr. Ferguson said.

"If we do not make progress on these fronts, Australia will not grab that second investment pipeline of up to an additional A$230 billion," Mr. Ferguson said.

Many investors are betting that China will lead a recovery in commodities markets by taking action to stimulate its economy. In recent days, Beijing approved dozens of infrastructure projects such as subways and highways, which Nomura estimated would add up to one trillion yuan (US$157.6 billion) of investment over four years. That news contributed to a mild rebound in prices of iron ore and Australian mining companies' stock.

China's economic slowdown is showing signs of reaching a bottom and the economy could pick up pace before the end of the year, said David Peever, head of Rio Tinto PLC's (RIO) Australian unit.

"The government [is] letting the handbrakes off. This is a good sign...and we should start to see a rebound in demand," Mr. Peever said.

--David Winning in Sydney contributed to this article.

Write to Ross Kelly at ross.kelly@wsj.com, Rhiannon Hoyle at rhiannon.hoyle@wsj.com and Robb M. Stewart at robb.stewart@wsj.com

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