The cost of "bunker fuel" for the cape-sized vessels that typically carry iron ore has fallen 39 per cent since January, with most of that fall  from the start of October.

After spending much of the past four years between $US600 ($723) and $US700 per tonne, prices for bunker fuel were fetching just $US387 per tonne this week.

If the price were to remain at these levels for one year, the savings for Australia's biggest iron ore miner, Rio Tinto, would be more than $US450 million.

That estimate is based on recent comments by Rio's iron ore chief executive, Andrew Harding.

Mr Harding said the company would be spending about $1.50 less on each tonne of iron ore it shipped, thanks to softer bunker fuel prices.

The company is on track to ship 300 million tonnes in 2014.

"It is not insubstantial at all, it is quite strong," Mr Harding said.

The world's biggest iron ore producer, Vale of Brazil, ships its product farther than Australian miners to reach Asia.

Vale executives estimated last week they were saving $US2 per tonne based on the changes in the bunker fuel price.

The change is also helping Australian exporters such as Fortescue Metals Group, Atlas Iron, and BC Iron. All have suffered in recent months under the low iron ore prices.

BC Iron managing director Morgan Ball said the cost of freight to Asia had fallen from between $7 and $8 per tonne to between $5 and $6 per tonne.

Atlas Iron noted on Thursday that it was enjoying "favourable cyclical cost opportunities" in both the cost of freight and the cost of diesel, which is also linked to the slump in oil prices.

BHP's iron ore division would also enjoy lower fuel prices, but given the company is a sizeable oil and gas producer, the net result of the oil price slide is more likely negative.

Platts managing editor, Asia Pacific shipping and freight, Pradeep Rajan said miners were enjoying the cheapest shipping costs in almost two years.

"The current slump in the bunker prices, on the back of Brent crude falling by over 40 per cent since the second half of the year, has helped mining majors charter cape-sized vessels at 20-month low rates out of Australia," he said.

Iron ore exports through Port Hedland fell to a five-month low in November.

Mr Rajan said that was adding to an oversupply of vessels and further lowering the cost of shipping iron ore to China.

"The easing of port congestions and the slowing down of iron ore exports out of Western Australia, coupled with an oversupply of vessels, are forcing ship owners to accept rates that would just about offer them a slight positive return," he said.

Most Australian iron ore miners also consume large amounts of diesel to power their mines and fleet, and are enjoying handy discounts on diesel prices, too.

Mr Harding said Rio consumed "considerably more than half a billion litres per year" of diesel on its Pilbara iron ore division alone.

However, he would not say how much money the company was saving at current diesel prices.

In urban areas of Australia, diesel prices have fallen about 8 per cent since the slump in the oil price.

Oil prices have nearly halved since hitting a June high above $US115 a barrel.

The fact that Saudi Arabia – the world's largest oil producer – has refused to cut production amid the price slide has been a major factor in forcing oil prices down, and the ramp up in supply is testing the resolve of United States producers, which are expected to be vulnerable should oil keep falling to the $US50 range.