A mix up at a Sydney voting booth has forced the NSW electoral commission to "isolate" a number of ballot papers.

A person who wasn't employed by the electoral commission began issuing ballot papers to voters at a booth, understood to be in the Sydney electorate of Strathfield, on Saturday.

A NSW electoral commission spokesman said the votes in question would be counted separately.

"The NSW electoral commission is aware that a person who was not employed by the NSW Electoral Commission issued ballot papers this morning due to an administrative error," the spokesman said in a statement to AAP.

The spokesman said it was not yet known how many votes have been affected by the mix up.

"The number of votes involved will be determined during counting, which starts after 6pm tonight," the spokesman said.