A person who wasn't employed by the electoral commission began issuing ballot papers to voters at a booth in the marginal 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.

Sitting Labor MP, Jodi McKay, told the ABC she understands up to three boxes of votes could be involved.

"The returning officer I believe let them in and they had five hours or so actually working with votes," she said.

"I understand that there are three boxes of votes which could determine the result of my seat which have been quarantined and may not be counted tonight."

Ms McKay holds the seat with just a 1.8 per cent margin.

Australian Associated Press