With the fate of Africa's largest nation in the balance in a vote on south Sudanese independence that opens on Sunday, a massive training effort has been under way to make sure it is seen as fair.

At centres across the vast region, south Sudanese of all ages have been undergoing instruction on what it takes to staff a polling station properly and what electoral observers should do to ensure it is seen to be done so.

After decades of civil war, views across the south are inevitably strong but election staff are taught they must be scrupulously impartial in the face of the vocal campaign of street posters and marches by independence supporters.

Morris Luate was a refugee in Uganda during the 1983-2005 conflict, but has returned home and is training polling station staff after himself was trained as an instructor by the South Sudan Referendum Bureau and the United Nations Development Programme.

"During the war I was young, I couldn't help my country. Now is my opportunity to do it," he said.

"We are facilitating the opinions of the community, the citizens of south Sudan. They know that their opinions are handled in the right way."

A similar massive operation was carried out to train the staff who ran centres where voters registered last month to a broadly positive response from foreign observers.

But trainer Abanzi Elisha Michaelson said polling itself should actually pose fewer difficulties after the logistical problems of compiling the electoral register.

"The one for the registration was a bit complicated but this one is straightforward, so I don't expect them to make many mistakes this time," he said.

Among the challenges facing polling station staff will be to ensure a largely illiterate electorate knows how to cast their vote without it being deemed spoilt.

An incorrect fold of the ballot paper risks leaving the indelible ink applied to voters' fingerprints on both yes and no sides, leading to its disqualification.

An umbrella group of civic organisations, the Sudanese Network for Democratic Elections (SuNDE), has been conducting a voter education drive in marketplaces and outside churches to make sure voters know how to make their opinion count.

They have also been training up a battery of observers to monitor the conduct of the vote.

Harriet Baka Nathan, one of more than 160 additional observers who underwent an instruction course with SuNDE in the regional capital Juba this week, explained the requirements set to ensure new recruits added to the transparency of the referendum.

"You must sign the non-partisan pledge and you must be Sudanese and 18 years of age and you must be trustworthy," she said.

SuNDE will be fielding more than 2,500 observers at polling stations across the south, 10 times as many as the European Union and the Carter Centre foundation of former US president Jimmy Carter put together.

Their local knowledge and reports on the conduct of the vote in the more far-flung villages is likely to be a major input into the overall assessment of the international missions.

"What we do when we meet the local observers is to ask them their views about the problem issues to see whether we have the same views," said Niall McCann of the European Union Observer Mission in Sudan.

"We are looking much more at issues about whether the voter education being offered is balanced," he added.

In the final analysis, it is the verdict of the observer missions that is likely to play the determining role in how the outcome of the referendum is seen by the outside world, whether or not it leads to the birth of a new nation this July.

It is not just the majority either way that is at issue but also the turnout -- the 2005 peace deal between north and south that provided for the referendum requires that at least 60 percent of registered voters cast a ballot.

If independence supporters, who include the south's autonomous regional government and its security forces, are seen to have herded voters into polling stations to ensure the quorum has been reached by the end of the week-long polling, any new nation risks being seen as the fruit of an illegitimate birth.