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A police statement described in the report said the substance had been taken to police forensic laboratories for examination.
"Ambulance officers have provided precautionary treatment to a number of employees," said police, as quoted in the Reuters report.
McDonald's Australia said in the Reuters report that only some of the suspicious envelopes sent to its stores contained the unidentified substance. However, restaurant managers across the country had been told not to open any mail until further notice, said the report.