SYDNEY - Iron ore exports to China from Australia’s Port Hedland rose 15 percent to 33.9 million tonnes in August from July, as BHP Billiton and Fortescue Metals Group operated at maximum production rates despite a softening market, figures released on Thursday showed. The August increase over July sent total iron ore exports from the world’s biggest iron ore export terminal to 39.2 million tonnes in August, a record for a single month and an 11 percent rise over July. Iron ore in recent years has replaced coal as Australia’s biggest source of foreign income, despite the price plunging as low as $44.10 a tonne in July - less than half of the price a year earlier. Iron ore has since rebounded to stand at $55.80 a tonne. Port Hedland, which handles about a fifth of the world’s seaborne iron ore trade, is used by BHP and Fortescue, the lion’s share of shipments headed for Chinese steel mills. BHP is lifting output by 6 percent to 247 million tonnes by next July, while Fortescue is running at an annual rate of around 165 million tonnes, making them the third and fourth highest producers worldwide after Vale and Rio Tinto .