By Rhiannon Hoyle and James Glynn

SYDNEY--Spending on resources projects in Australia has fallen at its fastest pace for 14 years, adding to the economic challenges facing Prime Minister Tony Abbott at a time when government revenue is shrinking and unemployment is edging higher.

The Australian government's commodities forecaster on Wednesday reported a sharper-than-anticipated drop in investment in mining and energy projects between May and October, as companies scaled back expansion plans and finished up major developments.

At the end of October, there were 63 mining and energy projects that companies had committed to building in Australia, worth a combined 240 billion Australian dollars (US$219 billion), according to the Bureau of Resources and Energy Economics, or Bree.

That was down from 73 projects worth some A$268 billion six months earlier, it said in its biannual report.

"This large drop in investment is the result of two records being set in the period--a record high for the value of projects being completed and the lowest value of new projects being sanctioned in the past decade," Bree said.

"Both measures are initial indicators that the Australian resources boom is now transitioning from the investment phase to the production phase," it added.

The slide was greater than expected, with the bureau having earlier this year forecast that investment would still total some A$256 billion at the end of 2013. It was the largest drop in overall resources investment since 1999, according to Barclays analyst Kieran Davies.

"There is a real risk that we're on the cusp of a massive decline in mining expenditure here," Mr. Davies said. The government's forecaster says investment in the Australian resources sector may slide by more than two-thirds over the next five years.

Alan Oster, Melbourne-based chief economist at National Australia Bank, said the data was confirmation that the slide in mining investment was happening at pace that's surprising most forecasters.

NAB expects mining investment to contract at rates of between 10% and 20% per year "for as far as the eye can see," said Mr. Oster, a former head of economic forecasting at the Treasury.

The figures highlight the challenge faced by the Australian economy, now the world's 12th largest, to find sources of growth to replace mining.

Australia's economy has slowed over the past year in line with weakness in China--the country's largest trading partner--and lower commodity prices, which has prompted mining firms to close mines, lay off thousands of workers and put on ice plans for new projects.

BHP Billiton Ltd. (BHP) Chief Executive Andrew Mackenzie has said the world's biggest mining company would scale back capital spending significantly in the coming years from a peak of more than $22 billion.

The RBA has cut interest rates eight times to a record low 2.5% to brace the economy against the mining slowdown and encourage growth. Australia's unemployment rate was 5.7% in October, compared with a rate of 5.4% at the start of the year.

Mr. Oster said that so far there was no evidence of a recovery in investment in non-mining parts of the economy, which may encourage policymakers to consider further interest-rate cuts early in 2014.

"In non-mining investment, we are seeing absolutely nothing," he said.

Government income has fallen in parallel with the mining slowdown. Ahead of a scheduled mid-year update on government finances in December, Treasurer Joe Hockey has warned of an ongoing deterioration in government revenue due to the soft economy.

He told reporters earlier this month to expect the budget to be "substantially worse" by the time of the update, compared with the last review released ahead of the last election in early September.

The central bank recently lowered its growth forecast for 2014, saying mining investment may drop off more sharply than expected, and that an elevated local currency was likely to hurt sales of manufactured products. It lowered its economic growth estimate for the upcoming calendar year to 2%-3% from an August forecast of 2.5%-3.5%.

The Bree data comes a day ahead of the release of official third-quarter business investment data by the Australian Bureau of Statistics, which is expected to further confirm the mining investment slowdown and show other parts of the economy struggling, despite recent data showing a strengthening in business confidence.

Write to Rhiannon Hoyle at rhiannon.hoyle@wsj.com

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