"Port inventories are still relatively high, so there's more downside to iron ore prices.": UOB Kay Hian analyst Helen Lau.

Iron ore has fallen below $US90 a metric tonne for the first time since 2012, after tumbling a further 2.1 per cent overnight amid slowing demand from China.

Ore delivering to the port of Tianjin dropped to $US89, taking its slide for the year to 34 per cent as mining companies continue to expand supply despite slowing growth in the world's second biggest economy.

Iron ore stock piles at Chinese ports are now sitting near record levels, putting sustained downward pressure on prices. Bloomberg reports that reserves at China's ports have climbed 31 per cent this year.

"Port inventories are still relatively high, so there's more downside to iron ore prices," UOB Kay Hian analyst Helen Lau told Bloomberg, predicting a floor of $US80 in the second half.

"During the summer season, you can expect the continuous slowing down of steel production. Steel mills have no interest in replenishing their stocks."

Global consultancy Wood Mackenzie has tipped several years of relatively weak iron ore prices as Australian produces attempt to increase their share of Chinese imports.

Wood Mackenzie has been forecasting average prices this year of $US107, falling to $US98 in 2015 then to a low point of $US90 in real 2014 terms, including costs and freight, in 2017.

Wood Mackenzie's comments came as Deutsche Bank downgraded its iron ore price forecasts for the next three years. It trimmed its 2014 benchmark price for iron ore fines by $US3 to $US113.10 a tonne and now expects a 10.3 per cent drop in 2015 to $US89.30.

Wood Mackenzie expects oversupply to be a feature in the market until about 2020, the duration depending on how quickly high-cost Chinese domestic ventures exit the market.