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The accident involved one of Rio's rail cars, which fell into two rail car dumpers at the Cape Lambert port.
In a statement, the miner said no one had been injured in the incident and the damage was being assessed to determine how quickly the dumper can be returned to operation.
The company also said it could take up to three weeks to restore operations to normal levels, however the reduced output would not have a long-term impact on the company's iron ore exports this year.
The company has not yet revealed the exact level of impact to its shipping operations, however.
Rio Tinto is targeting production of more than 190 million tonnes this year from its Pilbara operations.
The company also plans to expand its Pilbara capacity to 320Mt a year by 2012 and will expand its mine network with six additional mines and construction of 100Mt of additional port capacity.
Shares in the major have been battered this week and fell a further $1.52 this week to $111 after a 6% fall by close of trade yesterday.