Snickers and Mars Bar manufacturer MasterFoods has received another threatening letter from the extortionist behind the chocolate bar contamination scare, the Australian Associated Press reported.

A spokesman for MasterFoods said it had received a fourth threatening letter on Wednesday morning at its head office in Ballarat, Victoria.

MasterFoods would not outline the content of the new letter but said it did not require changes to the massive recall of Snickers and Mars Bars, which affects only New South Wales.

"In relation to the new letter, there was no information in it that requires a change of our current course of action as far as the recall is concerned," the company's spokesman told AAP.

The letter was not accompanied by a chocolate bar and had been handed to police for investigation, he said.

The fourth letter was received on the same day as scientific tests confirmed a Snickers bar sent to MasterFoods' head office last month was contaminated with a substance similar to a common pesticide.

MasterFoods last Friday announced a recall of tens of thousands of Snickers and Mars Bars after it received a third threatening letter from a person making demands of a third party.

The author of the letters claimed seven Snickers and Mars bars in the Sydney area had been laced with poison.