Secretary-general Sutthichai Sungkamanee said the gang would send letters to companies telling them their VAT invoices would be inspected.
It will then contact the companies and demand bribes, threatening to take action against the companies if they refuse to pay.
Mr Sutthichai advised companies which had received such letters to check with the department first. If it is a scam, the department can trace back to the swindlers and take legal action against them.
Among the telltale signs that a letter might be fraudulent is that it does not have the address or the wrong address of the department, Mr Sutthichai said, referring to the letters sent in March by the gang.
Besides, it uses the department's mark as the letterhead instead of the Garuda emblem, the standard mark of all government letters.
As well, the sender code and the letter running number in the header are inconsistent. The department's head office is identified as the sender but the letter number is Kor Kor 0709, the code used by the department's Region 5, not the head office.