The stock declined 2.3 percent to 107.50 rand at the end of business in Johannesburg, its lowest value since July 2010.
Dino Melaye had claimed a month ago that MTN may have illegally moved $14bn out of Nigeria, claims which the firm categorically denied.
However on Wednesday Senator Melaye, citing early findings from an investigation into the allegation said the amount is much more than earlier suspected.
“We have realized from preliminary investigations that it is actually outrageously higher than” the original figure, he said.
Senator Rafiu Ibrahim, the chairman of the committee that’s leading the inquiry added that international forensic experts, accountants and lawyers are examining documents dating back 16 years.
Four months ago MTN agreed to pay a 330 billion naira ($1.1 billion) fine in cash to the Nigerian government and get listed on the stock exchange. These were sanctions imposed on the firm for failing to disconnect unregistered customers in Nigeria.