Some debit cards have also been Visa enabled, in which case you could press the credit button, rather than the savings or cheque button to debit your account.
In future, at Woolworths, if you wish to use a debit card, you will have to press the cheque or savings button. The credit button presumably wont work.
As I've mentioned earlier, this isn't an ideal solution for a lot of people. Visa Debit cards are attached to a "Savings" account. Very few savings accounts are fee free, and most have limits on the number of transactions per month before additional charges apply. Visa "Debit" transactions are generally considered as free transactions and not subject to the transaction limit. Which is why this is a preferable way to pay for a lot of people.
I really don't like what Woolies is doing. Yeah I have shares in them, so really should be congratulating them for saving some costs - but there are many sections of society who *want* to pay via Visa (or Mastercard) Debit, and Woolies should respect that.