Masterfoods, a management organisation that represents private US company Mars in Australia and New Zealand, said that a threat has been made to poison a member of the public using tampered chocolate bars.
The company has begun the recall of tens of thousands of Snickers and Mars bars from over 5,000 stores in New South Wales as a result.
"Due to the very serious nature of the tampering threats, we are instigating an immediate withdrawal of Snickers and Mars from all NSW retail outlets," company president Andy Weston-Webb said in a statement.
"No one should eat a Mars or Snickers bar that they have bought. Please destroy it ... it is not safe to eat Mars or Snickers bars," he added.
Police tests indicated that a Snickers bar posted to MasterFoods' head office last month had been contaminated with an unidentified substance. This was followed by a letter this week in which the author claimed to have contaminated seven Snickers and Mars bars in the Australian capital, Sydney.
"There is a foreign contaminant definitely within the bar, as to what that substance is, chemically or physically, we are not aware," said a New South Wales police spokesperson.
Australian police have yet to find evidence that the threat has been carried out.