BHP Billiton's London-listed stock has sunk to a six-year low as the iron ore price marks a retreat back toward $US50 a tonne.

At the end of the latest session, benchmark iron ore for immediate delivery to the port of Tianjin in China was trading at $US50.70 a tonne, down 2.8 per cent from $US52.10 a tonne.

The red session was the first of significance since the commodity hit a 10-year low of $US44.10 over two weeks ago and again puts mid-tier and junior miners under the pump as all miners aside from BHP, Rio Tinto and Vale are likely clinging to breakeven prices upwards of $US45 a tonne.

The action followed a disappointing market update from BHP Billiton that pointed to significant writedowns that would likely drag its profit to its lowest level in 10 years.

While the giant miner's output largely topped expectations, the flagging of around $US5bn ($A6.7bn) in writedowns caught some by surprise.

Hurting the iron ore price specifically was news iron ore production would expand at a greater rate than expected.

BHP's WA iron ore production came in at 254 million tonnes over the year, beating guidance of 250 million tonnes, while next year's 270 million tonne forecast was above analyst expectations.

The development comes amid an already oversupplied market that soon needs to fit in supply from Gina Rinehart's giant Roy Hill mine.

While BHP lost 2.1 per cent in local trade on Wednesday, the pain was significantly greater offshore as its UK-listed stock tumbled 5.7 per cent.

BHP wasn't the only miner listed in Europe to be feeling the pain as Anglo American slumped 4.6 per cent to a 13-year trough and Glencore dived 5.4 per cent to its lowest mark since listing in 2011.

BHP's largest local rival, Rio Tinto, backtracked 3.6 per cent, as softer gold, oil and copper prices added to the downbeat mood.

Across the Atlantic, stock of the world's largest iron ore miner - Vale - retreated 2.9 per cent.