Roy Hill Holdings said on Monday a contractor employee reported a bad taste and unusual discolouration inside an item of fruit at the mine's dining hall.

The contractor was reviewed by onsite medics but showed no signs of any health issues.

The piece of fruit was independently tested by a certified laboratory in Perth and preliminary test results indicated that an instance of contamination had occurred.

The incident was reported to police, which have begun a formal investigation.

Roy Hill CEO Barry Fitzgerald said the company's top priority was the safety and wellbeing of its people.

"We have tightened our food security as a means of ensuring no further incidents occur; and are working closely with the police in their investigation, as this is now a police matter," he said.

Roy Hill said there were no further reports of contamination in any other food on site.

A police media spokeswoman said it was believed to be an isolated incident, though details of the case were being kept confidential.

The $A10 billion Roy Hill iron ore mine is more than two thirds complete and employs 7200 workers.

First ore on ship is due in September next year before the mine ramps up to 55 million tonnes per annum.

Roy Hill is owned by Hancock Prospecting, Marubeni Corporation, POSCO and China Steel Corporation.