Dunedin Hospital has hit capacity in the past 24 hours with the Southern DHB declaring it a "code black - critical situation".
Patients are being asked to avoid the emergency department (ED) if possible as the hospital has a high level of beds in use and faces a "heavy" demand on services.
Early on Wednesday morning, there were 18 patients in the ED awaiting a bed and only three beds available on the wards.
"We are doing our best to deal with this demand, but patients may experience delays and some operations have been postponed," says Southern DHB chief executive Chris Fleming.
"We are asking the public to go to their GP early, use the after-hours service and keep the ED for emergencies only."
The hospital has been declared a "code black" as part of a new plan which outlines actions staff need to take when the hospital is experiencing high demand or when capacity is reached.
"The plan has been put in action and a decision was made early this morning to declare the hospital in code black - critical situation," says operations centre incident controller Megan Boivin.
"This means that we have established a modified emergency operations centre."
Boivin says hospital staff have been working "extremely hard" to improve the flow of patients through the hospital.
"We realise that the pressure on staff has been unrelenting and we are urgently working to manage this high occupancy within Dunedin Hospital to allow patients to flow through the system in a timely manner including those who present to ED."
Fleming says the whole hospital is under pressure: "This is not just an ED issue but a whole of system issue."
He says the "code black" will enable a "consistent and coordinated response to manage" the situation.
"Without this response, our current processes will continue to be under pressure which impacts the patient journey and we do not want to see this continuing."