More than a third of every packaged product was marked down as much as 50 per cent for an hour after a "human error" in the company's IT department. A promotion for a specific range of products was accidentally applied across the board.
A Woolworths spokeswoman denied allegations hackers were behind the glitch which she said lasted an hour.
"These things happen . . . and it was sorted very quickly," the spokeswoman said.
"The company did not lose a significant amount of money."
She said the exact nature of price reductions was "complicated" and discounts depended on what customers bought.
It is believed some stores shut their doors until the issue was rectified but most continued trading. Members of an online forum identifying themselves as Woolworths staff debated the cause of the fault yesterday.