The Causeway Hospital in Coleraine has put in place a plan to deal with sustained pressure on its emergency department.
The Northern Trust said it was working with the Ambulance Service to divert patients away from the Causeway to Altnagelvin and Antrim Area Hospitals.
Extra staff have been brought in to deal with the increased pressure at the Coleraine facility.
The trust has asked people not to attend the A&E if they do not have an emergency.
East Derry MLA David McClarty told the BBC that he has been contacted by several concerned patients. When he raised those concerns with a senior member of staff, he was told that there were patients lying in trolleys in A&E and that there was considerable pressure on staff - who were working way beyond the call of duty.
He says staff described the Causeway to him as being like a third world hospital, the worst they had seen in 20 years in the NHS.
Mr McClarty also questioned the response from the hospital, saying he believes it has not been adequate to address the seriousness of the situation.