Altnagelvin Area Hospital declared a major incident and implemented a full 999 divert yesterday.
It is understood the Derry hospital's A&E unit was under pressure due to the number of patients.
Emergency departments in hospitals across the north experienced a similar spike in the number of people seeking medical help.
The Ulster Hospital warned of delays due to being "extremely busy" on Monday while the Belfast Trust tweeted that its emergency departments in the Royal Victorial Hospital and Mater Hospital were dealing with "large numbers" of patients.
"We have large numbers attending the RVH & Mater Emergency Dept and patients may have to wait. Those who are seriously ill will be seen first"
A spokeswoman for the Western Health and Social Care Trust said the major incident and 999 divert had not been in place for long at Altnagelvin and the hospital's A&E department was "back to normal" soon after.
In a statement the trust said it had "activated its business continuity measures" before Christmas which supported its work "through the outbreak of Norovirus".
"02 January 2017 proved a particularly difficult and challenging day and the Emergency Plan was enacted for a period of time to deal with a pressurised period in our ED. This measure was stood down shortly afterwards.”
We have large numbers attending the RVH & Mater Emergency Dept and patients may have to wait. Those who are seriously ill will be seen first
— Belfast Trust (@BelfastTrust) January 2, 2017
As of noon today the longest wait was 138 minutes at the South West Acute Hospital in Enniskillen followed by 71 minutes at the Causeway Hospital in Coleraine and 65 minutes at Daisy Hill Hospital.
On new year's eve the Northern trust reminded people that EDs are for people who require "the highest level of emergency care".
