Handbook lays out pay rates for any extra work
The rules on overtime are set out in a 12-page document which forms part of the Northern Ireland Civil Service handbook.
It defines overtime as "hours worked at management's request in excess of conditioned hours".
It adds: "These hours are voluntary in nature, except where they are part of the conditioned overtime for a post.
"Authorised overtime will, as far as possible, be worked on Monday to Friday. Overtime should only be worked at weekends if circumstances clearly justify this as necessary."
There are two scales of payment for overtime accrued Monday to Saturday.
For all grades up to and including EO1 level (ie junior manager) and equivalents, there is an hourly payment rate of time and a third. From staff officer grade to Grade 7 level (ie senior manager) and equivalents, there is an hourly payment of plain time rate.
For staff unexpectedly called into work outside normal hours, a minimum three-hour credit is counted towards overtime, even though the work may have taken less than three hours.
Staff called out in an emergency, where the work exceeds two hours, receive a callout bonus credit of one hour in addition to the hours worked.
Workers in grades eligible for overtime will receive a premium payment of half the plain time rate for attendance between the hours of midnight Friday and midnight Saturday. Minimum overtime credit or call-out bonuses do not apply in this case.
Staff in overtime grades (except those for whom the retention of special conditions has been agreed) whose pay does not include an element for Sunday attendance will be compensated at the rate of double time for attendance on Sundays.
The full overtime document can be found on the Department of Finance website.