Riek Machar's arrival in Juba is the first time the former Vice-President has returned since fleeing at the beginning of the country's civil war, two and a half years ago.

Doves were released at the airport when Machar, dressed in a casual shirt, disembarked a UN flight that had left from Gambella in Ethiopia.

Addressing the media at the airport, Machar said he hopes his return would "make sure that peace breaks out all over the country."

Securing peace, stabilising the economy and helping the country's internally displaced people were his top priorities, he said.

A peace agreement signed between the warring parties in August last year agreed that Machar would be reappointed as Vice President upon returning to the capital.

Sporadic fighting however continues throughout the northeast African country.

The re-installation of Machar is significant given President Salva Kiir originally sacked him from the position in July 2013.

Full-blown civil war broke out six months later.

"We're back to the status quo," says Abraham Awolich, an analyst with the local Sudd Institute think tank.

"That is not a good thing because the status quo is what led to the current crisis," he says.

Machar was due to return last week but was delayed when the government refused landing permission for his planes from Gambella in Ethiopia, alleging the rebels were bringing more troops and weapons than agreed.

His arrival comes days after an internationally backed ultimatum expired calling for the opposition movement's chairman to return by last Saturday.

The first shots rang out on 15 December 2013 between Presidential Guards at Army Headquarter barracks in Juba, pitting mostly Dinka troops allied to President Kiir against mostly Nuer troops supportive of Machar.

The next day President Kiir announced on national television that Machar had staged a coup. An African Union report dismisses the coup narrative, pointing instead towards disagreements, confusion and suggestions of mutiny within the Guards.

Fighting spilled into the streets and erupted into a three-day systematic roundup and mass killing of ethnic Nuer citizens and soldiers, according to the AU report.

Machar fled the city to set up and command the opposition from the bush. The conflict quickly spread throughout the country, largely splintering along ethnic tribal lines.

Subsequent investigations by the UN and the African Union found that both Government and opposition forces and their allied militia have committed war crimes.

Conservative estimates put the death toll at 10,000. Two million people have been displaced, the country is economically crippled and extreme food insecurity bordering on famine in parts, plagues the country.

The Australian government's travel advice to South Sudan is "Do not travel."