Staff at one of the affected stores at Sydney's Town Hall confirmed the outage meant customers were left with no choice but leave without purchases.
Stores temporarily closed in response to the outage.
"Registers are offline so customers are standing in queues with baskets of food that they can't buy", she said.
A woman named Rose, from Hoppers Crossing in Melbourne, told 3AW customers were offered lollies as they were asked to leave.
More than 20 shoppers were reportedly turned away by the security guard.
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According to multiple reports, Woolworths supermarkets around Australia are experiencing a technical issue that prevents them from processing any transactions. The registers are now back online in all our stores.
Seven News in Adelaide is reporting that some customers at a Woolworths store in Adelaide's west got their groceries for free after the nationwide outage.
INITIAL: MAYHEM has erupted at Woolworths stores across the Mackay region as a national technical outage shuts down stores.
The supermarket giant has not yet revealed the cause of the outage.