Australian iron ore exports almost certainly shrank in the first three months of 2015, suggesting the 28 per cent fall in iron ore prices over the same period was influenced more by weak demand in China than supply growth from Australian miners.

Fresh data published by Rio Tinto on Tuesday shed new light on the iron ore price collapse. It comes as BHP Billiton prepares to reveal higher iron ore exports in its March quarter results on Wednesday.

After a period of intense political pressure over its strategy to continue investing in iron ore export growth, Rio's March quarter results revealed a 12 per cent reduction in iron ore exports compared with the December quarter. Wet weather and a train derailment were blamed for the shortage.

The shipment of 72.5 million tonnes from its global operations was weaker than most analysts had expected, while the amount of iron ore mined was also weaker than the two previous quarters at 74.66 million tonnes.

Rio's result comes after Arrium, Grange Resources and Fortescue Metals Group also revealed lower iron ore shipments in the March quarter than the December quarter. Across the four companies, the reduction was close to 11 million tonnes.

Mount Gibson Iron may also reveal reduced export volumes, given its Koolan Island mine has been shutdown, while Chinese domestic iron ore production has shrunk four months in a row to be back to its May 2012 level.

UBS analyst Glyn Lawcock said iron ore exports from Australia's biggest bulk commodity port, Port Hedland, had been flat since the September quarter.

"We haven't actually seen any growth in Australian exports now for six months and I think what you can deduce from that is demand has probably been a bit weaker than everyone thought," he said.

"The first two months of the year are always very slow in China; it is winter there and Chinese New Year so therefore construction activity is quite poor always."

Mr Lawcock said weak steel prices in China might have also affected iron ore prices, which fell from $US71.49 per tonne on January 6 to $US51.35 per tonne on March 31.

"Chinese steel prices lead iron ore prices, not the other around, that is my personal view. If you look at steel prices in China, they have been falling, so steel mills, therefore, are not prepared to pay as much for the iron ore," he said.

The comments came on the same day that the World Steel Association predicted steel demand would shrink in China in 2015 and 2016, and less than a month after BHP's former iron ore president, Ian Ashby, said big miners had to concede that Chinese steel growth was not going to continue growing until 2030, as they had long predicted.

Another piece of the iron ore price puzzle will be revealed on Wednesday morning when BHP gives its March quarter results.

Analysts suspect it has exported about 63 million tonnes during the period.

The world's biggest iron ore miner, Vale of Brazil, will publishes its production numbers on Thursday morning.

Despite the weaker export numbers, Rio maintained its full-year export guidance for iron ore at 350 million tonnes, which if achieved, would represent a 16 per cent increase on 2014.

The miner may need to export stockpiles to achieve that target.

Iron ore prices have improved over the past week. They were $US51.57 per tonne on Tuesday morning.

Despite flattening over the past six months, iron ore supply should grow over the next 12 months, as Gina Rinehart's Roy Hill mine comes into production and the big miners continue expanding.

Aluminium overtook copper as Rio's second-biggest money-spinner at the February full-year results and production of the lightweight metal was 809,000 tonnes in the March quarter.

That was largely in line with the 817,000 tonnes expected by analysts.

Copper production was better than expected at 144,000 tonnes during the quarter.

Morgans senior resources analyst Adrian Prendergast said Rio's strong performance in coking coal was one of the highlights from the result.

Production was 22 per cent higher than the December quarter and 10 per cent higher than the March quarter in 2014.

Rio slightly downgraded its production guidance for titanium dioxide and diamonds. However, neither is likely to move the market.