* China iron ore demand growth seen falling to single digits
* BHP sticks to $10 bln iron ore expansion plans
* Rio to lift Australia iron ore output to 283 mln T by 2013
* BHP mining iron ore at rate of 165 mln-170 mln T/yr
* BHP sees current floor for ore price at $120/T
* Comments dent Aussie dlr, BHP stock dips
By James Regan and Rebekah Kebede
PERTH, March 20 (Reuters) - Australian iron ore miners, key beneficiaries of China's modern-day industrial revolution, on Tuesday signaled demand growth was finally slowing in response to Beijing's moves to cool its economy.
BHP Billiton, the world's biggest miner, said it was seeing signs of "flattening" iron ore demand from China, though for now it was pushing ahead with ambitious plans to expand production.
Rival Rio Tinto said it too was sticking with plans to raise capacity from its huge mines in Western Australia's Pilbara iron ore belt, betting on a soft landing for the Chinese economy.
"The (Chinese) economy is shifting, it's changing. Steel growth rates will flatten and they have flattened," Ian Ashby, president of BHP's iron ore division, said ahead of the Global Iron Ore & Steel Forecast Conference in Perth.
China's demand for iron ore, a key steelmaking ingredient, will slow to single digit growth, but the country's annual steel output will still rise by some 60 percent by 2025, Ashby said.
The news knocked the Australian dollar, which fell a fifth of a U.S. cent to $1.0595 right after the comments. The forex market is very sensitive to any hint of softening demand in China, given it is Australia's single biggest export market.
"What is concerning is the potential impact of single-digit demand growth for iron ore," said Stan Shamu, an analyst at IG Markets.
"Iron ore demand from China makes up a bulk of our exports and this will impact terms of trade. This shows that terms of trade are peaking and likely to flatten down the line."
Chinese demand for iron ore has been the driving force behind years of expansion work by the world's biggest mining companies. More than 100 million rural Chinese are projected to settle in towns and cities in the next decade, requiring unprecedented amounts of steel for housing and infrastructure.
Earlier this month, however, China cut its 2012 growth target to an eight-year low of 7.5 percent, fuelling caution about demand for resources.
The miners stopped well short of declaring an end to China's commodities boom, but dimmed their outlooks.
Rio, BHP and other big miners have been pursuing a strategy of running at full production and expanding capacity in long-life and relatively low-cost commodity assets compared to the selling price of ore, banking on squeezing out higher cost producers.
BHP was sticking with its $10 billion iron ore expansion plan and was mining ore at a rate of 165 million to 170 million tonnes per year, Ashby said.
That is above its production guidance of 159 million tonnes in fiscal 2012 ending June 30, maintaining the company's No.3 global ranking in iron ore behind Vale and Rio Tinto.
"The size of the pie is really big, so any percentage increase is a significant number," Ashby said.
Analysts agreed even single-digit growth in Chinese ore demand should be enough to spur miners to push ahead with production expansion plans.
"Certainly the rate of growth in Chinese demand is slowing, but the growth is from an ever increasing base so the number of iron units required continues to rise," said Paul Gray, analyst at Wood Mackenzie, who sees the seaborne iron ore market staying tight through 2012 and 2013.
Rio was also sanguine about the slowing growth.
"Although the rate of GDP growth in China is more immediately slowing, we remain confident, on the basis of the figures we have seen, of a soft landing, with solid growth for this year," David Joyce, Rio Tinto's managing director of expansion projects, said in a speech.
BHP saw the current floor for global iron ore prices at $120 a tonne, based on the estimated highest cost of production inside China, Ashby said.
Iron ore has sold for between $130 and $147 a tonne over the last four months, which mega-producers such as Rio Tinto have said is high enough to warrant investment in new mines.
Rio Tinto has mapped out plans to lift its overall annual production of iron ore in Australia to 283 million tonnes in 2013 from 225 million now by digging new mines and expanding existing ones.
Global iron ore demand is set to double to around 3.5 billion tonnes a year by 2030, with Chinese appetite for the steel-making material continuing to drive the market, albeit at a slower pace, according to Perth-based Intierra Resource Intelligence.
Shares in BHP were little changed in Australian trade, easing 0.1 percent in a broader market down 0.3 percent. Rio shares were off 0.5 percent.