Grocery giant Woolworths has scrambled to cancel over $1 million worth of shopping vouchers after a massive leak of customer data, in which it mistakenly emailed the redeemable codes of 8000 gift cards containing the customers' names and email addresses.

Fairfax Media has obtained a copy of the email which contained an excel spreadsheet with the names and email address of thousands of customers and a downloadable link to 7,941 vouchers, worth a total of $1,308,505. It is understood the spreadsheet was emailed to more than 1000 people, all of whom could access the gift card codes and immediately begin shopping.

When Jason Wang checked the balance on his gift cards he found some had already been spent in Sydney stores.

As a result of the data leak, customers reported they had logged onto the Woolworths site on Saturday only to discover their vouchers had already been spent.

The data breach, which was discovered on Saturday morning, occured after customers purchased the vouchers from the online savings site Groupon, which ran a deal last week offering BIG W eGift cards, valued at $200 and $100, at a 7.5 per cent discount. The cards were redeemable at Woolworths online and in store, Big W stores, and Caltex petrol stations.