David Shearer has told the UN Security Council that things are going from bad to worse in South Sudan.

Shearer, former leader of the Labour Party, left his role as MP in Mt Albert last year to take the role as the United Nation's head of mission in the war-torn country.

He is now in New York giving an update after three months on the job, and has told the Security Council things are deteriorating.

South Sudan separated from the rest of the country in 2011 but was now riven with ethnic violence.

He said appalling roads and the need to work through official and unofficial checkpoints throughout the country massively delayed the delivery of aid throughout the country, saying a 1000km journey could take two weeks.

He said the UN Mission (Unmiss) protected hundreds of civilians around the country.

"I have no doubt that thousands of civilians would not be alive today if not for the presence of of Unmiss."

The security, economic and humanitarian situation had worsened dramatically since last year and the conflict had intensified, Shearer has told the council.

He said more than 15,000 civilians fleeing violence and sought shelter from Unmiss in recent months.

Multiple witnesses reported the Sudan People's Liberation Army had shot many people after asking their ethnic groups, he said.

Government forces had also torched houses, hospitals and schools, and targeted men and boys along ethnic lines. Sexual violence was also rife, Shearer told the council.

He said a third of South Sudan's population was displaced or are refugees.

"This is a manmade disaster. Three years of conflict has eroded their livelihoods, prevented them from harvesting and caused cattle to be looted on a grand scale."

However, Shearer also paid tribute to aid workers. Recent attacks have killed 10 aid workers as humanitarian bases are looted and attacked.