David Shearer says the future of famine and war that South Sudan faces is grim, but there is hope.

The former Labour leader now heads the United Nation's mission in South Sudan.

Mr Shearer told Mike Hosking it's mainly conflict - not drought - that's caused the famine, and he's hopeful the conflict can be reconciled as it has been in other parts of the country.

"I've been in lots of countries in the past that have been lost causes and they've pulled themselves through. It's the newest country in the world and it's fought for a long to get its independence."

Mr Shearer told Mike Hosking is causing people to flee their homes, and their livelihoods.

"In this particular area where the famine is being declared there' about 100,000 people who are basically hiding in swamps and taking refuge in other places. Without food they're effectively slowly perishing."