Thousands of people have been caught up in Woolworths whoopsie.

The supermarket wrote to winners of a promotion, awarding them 4000 reward points - which equals $30 worth of groceries.

But it sent about 80,000 emails, instead of 1000.

It's apologised and offered 500 points compensation to those wrongly emailed.

Auckland University marketing professor Mike Lee says it's understandable if people are upset.

"It sets up that expectation, and then to take it away... even if they were to offer something at half-price, people are going to be left with a bad taste in their month."

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