Rio Tinto's shares have fallen by more than 2 per cent in early trade after it posted its first ever loss.

Rio Tinto's announced a full year net loss for 2012 of almost $US3 billion ($A2.91 billion) after the Australian sharemarket closed yesterday.

The mining giant was dragged into the red after making $US14 million ($A13.59 million) of writedowns.

Its underlying profit fell 40 per cent to $US9.3 billion ($A9.04 billion), but still beat market expectations.

Rio's shares were $1.59, or 2.21 per cent, lower at $70.48 at 9.30am.

Bell Direct equities analyst Julia Lee said it appeared profit takers had moved in on Rio Tinto's shares, which have risen 43 per cent since last September.

"There's a lot of optimism that's already been priced in," she said.

"It's actual underlying (profit) number beat expectations.

The dividend was outstanding, it increased by 15 per cent.

The commentary was positive as well.

"So I think it's just a bit of profit-taking, especially given that we saw such a good performance on the Australian market yesterday which was followed by weakness offshore."

Rio Tinto's new chief executive Sam Walsh told investors he would slash costs and sell poorly performing assets after the miner posted its first-ever net loss.