Woolworths has apologised to thousands of customers after mistakenly telling about 79,000 people they had won a competition.

The email for the Big Night In prize was meant to be sent to about 1000 winning customers.

Instead, 79,000 extra so-called winners were sent the same email - awarding them 4000 points towards the supermarket's Everyday Rewards card system, which would equal $30 worth of groceries.