The Canterbury District Health Board is urging people to only visit Christchurch Hospital's emergency department in an emergency.
It says a high number of patients and staff sickness mean people will likely be in for a long wait if their condition is not urgent
The chief medical officer Helen Skinner said the hospital had 527 patients overnight - 105 people were being treated in the emergency department.
The hospital is asking people to call their GP first.
Christchurch also has three urgent care clinics that can treat a range of conditions.