Supermarket giants Coles and Woolworths expect to spend hundreds of millions of dollars more to repay staff the companies underpaid, following a legal judgement experts say could have wide-reaching implications.

Woolworths has flagged potential additional costs topping $500 million after tax, while Coles has put its preliminary estimate at up to $250 million.

The development could see the total cost of the underpayment scandal soar past $1 billion, with Woolworths having already repaid $330 million to thousands of staff, while Coles has repaid $31 million so far, and had set aside a further $19 million before today's extra provision.

On Friday, the Federal Court handed down a judgement on the historical underpayments of employees at the two major supermarkets, affecting nearly 30,000 em

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