Sydney businessman Salim Mehajer will have to serve 11 months behind bars over an electoral fraud plot with his sister.

Before reading out her reasons in the Downing Centre local court on Friday, the magistrate Beverley Schurr said she would be jailing the 32-year-old for 21 months and ordering him to be released on a good behaviour bond after he served 11 months.

She had previously found him guilty of 77 charges relating to a joint criminal enterprise with his sister Fatima to influence the 2012 Auburn city council vote.

Fatima Mehajer was given a two-month suspended jail term and placed on a $500 nine-month good behaviour bond on Friday.

Schurr described the offending as “towards the top of the scale of criminality for this kind of offence”.

She said the pair had engaged in a joint criminal enterprise to add voters who did not live in the Auburn council electorate to the rolls. “Their text message exchanges on the day constituted direct evidence of the planning and intention,” she said.

The judge found that the applications to the Australian Electoral Commission came from only two computers, one registered to a Mehajer family company and the other registered to the address where many members of the Mehajer family resided.

“The efforts of the two defendants succeeded only in part. Vigilant staff at the Australian Electoral Commission stopped processing the electronic applications late in the afternoon when they became suspicious of the number of applications and the repeated use of certain addresses,” Schurr said.

“However, 18 of the applications had already been processed before the freeze was imposed and the names of 18 ineligible persons had been placed on the electoral roll at the false addresses.”

She said votes were cast in the 2012 Auburn council elections in the names of all 18 ineligible electors.