Woolworths has upped its estimated staff underpayments to $315 million and says the final amount could still be higher once a review into the nine-year scandal is complete.

The supermarket giant – which also owns Big W – on Tuesday said it would exceed its initial guess of $200 million to $300 million in salaried staff underpayments since 2010, with a review having now covered five years of data.

The previous estimate was based on two years worth of information.

Woolworths’ also announced first half profit slipped 7.7 per cent to $887 million.