The two highly infectious bugs have swept through the region in recent weeks, as health services struggled to cope with the nearly simultaneous outbreaks.
It is thought the vomiting bug may have peaked, but the Public Health Agency says it has observed a "marked increase" in the number of flu cases over the past two weeks.
The out-of-hours service for counties Londonderry, Tyrone and Fermanagh received 13,000 calls in December alone.
Over half of the calls to Western Urgent Care were made during the Christmas holiday period.
A similar pattern was observed in other parts of Northern Ireland, with extra staff drafted in to cope.
"The large percentages of the calls we are getting are for flu and flu-like symptoms or respiratory tract infection, and as well as that there is the normal sickness or norovirus", Eugene Dunn, from Western Urgent Care, told UTV.
"We've been dealing with a lot of that over the Christmas period or end of year period."
Doctors warn there will continue to be a rise in the number of people contracting flu in the next few weeks, especially children and young teenagers.
"Most of the older population in Northern Ireland has been vaccinated against the flu so it tends to be the younger patients who haven't been vaccinated," Dr Tom Black from the British Medical Association explained.