Last year I asked at Piraeus bank if I could use my credit and debit card when on holiday outside Greece. At that time I was told I could withdraw cash at ATMs on the same basis as here and spend up to €600 daily in shops, restaurants, hotels etc., and I could request a higher limit if required.
More recently I used my debit card for an on-line purchase from a supplier outside Greece. I don't know but that may depend on how much money has been transferred into the bank from outside Greece - i.e. under the same rules as withdrawing cash here in excess of the two-weekly limit.