The error in checking the electoral roll came during one of busiest days so far at the Fitzmaurice Street pre-polling centre.

Voters were turned away temporarily on Wednesday morning, with lines snaking out the door as NSW Electoral Commission staff reverted to backup system.

Labor candidate Dan Hayes, who was handing out how-to-vote cards on Fitzmaurice Street during the outage, said some voters were asked to"go and get a coffee" during the outage.

"They have been having problems on and off all day," he said.

"The staff are doing the best job they can but there should be a better backup system so people can vote when they want to."

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A NSW Electoral Commission spokesperson said all pre-polling centres would remain open while the commission worked through the statewide issue.

"The NSW Electoral Commission has detected a performance issue with its electronic roll mark-off system. This has resulted in intermittent outages of that system at early voting centres," the spokesperson said.

"No voting centres are closed. The issue is currently being examined and is expected to be resolved shortly.

"There are backup arrangements in each early voting centre to allow voters to have their names marked off the roll. Activating those arrangements may result in a delay for some electors in casting their vote.

"We apologise for any inconvenience."

The online iVote system has also been closed for system maintenance after a potential security weakness was identified.

Early voting opened on Monday for the NSW general election on March 23.

More than 30 per cent of Wagga voters cast their ballot at a pre-polling centre during the September byelection.