Dunedin Hospital has hit capacity and the Southern District Health Board has declared a "code black" as it calls on the public to avoid the emergency department if possible.

SDHB chief executive Chris Fleming said this afternoon that the hospital was doing its best to deal with heavy demand at a time of high occupancy, but patients could experience delays and some operations had been postponed.

"We are asking the public to go to their GP early, use the after-hours service and keep the ED for emergencies only, Fleming said.

But the issue was ''a whole of system" issue and not restricted to the emergency department, he said.

In line with the Dunedin Hospital's trial ''escalation plan'', the hospital had declared what the health board called a code black, meaning the situation was deemed critical.

Emergency operations centre incident controller Megan Boivin said the declaration of a code black meant the hospital had established a modified emergency operations centre.

Early this morning, there were 18 patients in the ED awaiting a bed, and only three beds were available on the wards.

Staff had worked hard to improve the flow of patients through the hospital and the situation had improved, she said.

But pressure on staff was "unrelenting" and staff continued to urgently work to manage the high occupancy within the hospital to allow patients to flow through the system in a timely manner, including those who present to ED, she said.