It is now compulsory to wear a mask in shops and other enclosed public spaces in Northern Ireland.
The decision was taken by ministers last week as part of efforts to suppress coronavirus. Infections in NI have risen three-fold since early July.
Stormont said it wanted enforcement to be light-touch but warned fines of up to £60 were possible.
The Department of Health said it would be an offence to breach the law without having a “reasonable excuse”.
Breaches could attract a fixed penalty notice of £60 (reduced to £30 if paid within 14 days), it added.
The amount increases if there are further offences.
First Minister Arlene Foster said the executive would have “failed” in explaining the importance of people wearing face coverings, if the situation arises where fixed penalty tickets are being issued.