Iron ore is a buyers' market and prices will probably extend declines this half as low-cost supplies expand, according to Goldman Sachs Group.

Recent weakness seen in Australian shipments is only a temporary lull on the way to further expansion, analyst Christian Lelong said in a note received on Tuesday. When operations among major Australian producers are back at full strength, and a new mine at Roy Hill opens in a couple of months, the downward pressure will probably resume, he said.

Iron ore sank to its lowest since at least 2009 this month as the largest producers including Rio Tinto Group and BHP Billiton boosted output into an oversupplied market and commodities from oil to copper extended losses. While Goldman described Australian exports this month as sluggish, the bank said freight activity in Brazil was headed back to record levels. The countries are the two biggest shippers of the ore.

"Stock levels may start to grow modestly in the months ahead as supply growth accelerates once again but, in a buyers' market, this is likely to come at the expense of further price declines," Lelong said.

Ore with 62 per cent content delivered to Qingdao rose 2.1 per cent to $US53.45 a dry metric ton on Tuesday, the highest in three weeks, according to data from Metal Bulletin Ltd. Prices retreated to $US44.59 on July 8, the lowest level in at least six years, and have lost 25 per cent this year.

While Goldman didn't give a price estimate in the latest note, the bank said in a July 20 report that it expected prices to drop over the next four quarters to $US44 by the April-to-June period of 2016 from $US49 a metric ton through September.

"The prospects for iron ore seem rather bleak," said Wu Zhili, an analyst at Shenhua Futures in Shenzhen. "There's more seaborne supply to come, especially from Australia. However, growth in supply won't be as strong as last year because China's demand for imported ore has begun to fall."

Miners' shares were mixed in Sydney on Tuesday as iron-ore futures advanced in China. Rio climbed 0.2 per cent to $A51.28, while BHP lost 0.6 per cent and Fortescue Metals Group rose 2.3 per cent. The three are Australia's biggest producers.

"Since the start of the month, exports have been particularly disappointing in Australia, where scheduled maintenance at some terminals appears to be affecting shipments," Lelong wrote. Still, that's been partly offset by a strong rebound in Brazil, he said.

Exports from Australia will expand 10 per cent next year to 824 million tons, according to the country's Department of Industry and Science. Shipments will be lifted by operations at Roy Hill, the new mine in the ore-rich Pilbara that's backed by billionaire Gina Rinehart and set to start exports this half.