Coles has insisted all of its managers clock on and off when they work, after discovering they are owed $20 million in overtime as the supermarket giant became the latest of dozens of businesses caught out for underpaying workers.

It is now under investigation after it revealed on Tuesday that it had set aside $15 million in backpay for hundreds of salaried managers at its supermarkets and liquor stores, plus $5 million in interest and costs, as a result of underpaying them over six years.