On the eve of the siblings' criminal trial in the Local Court, Ms Mehajer struck a deal with Commonwealth prosecutors to plead guilty to 77 counts of giving false or misleading information to the Australian Electoral Commission.

Clutching her baby daughter, Ms Mehajer appeared briefly in court on Thursday to confirm the deal before being excused by magistrate Beverley Schurr.

The offences, under the Criminal Code, carry a maximum penalty of 12 months' imprisonment.

Mr Mehajer, who was elected to Auburn Council in September 2012 and later became deputy mayor, has pleaded not guilty to more than 100 electoral fraud offences.

On the first day of Mr Mehajer's criminal trial, prosecutor Jeremy Rapke, QC, told the Downing Centre Local Court that Mr Mehajer had "engaged in acts of electoral fraud designed to enhance his chances of winning the election" in 2012.

Mr Rapke said Mr Mehajer was elected to council with just 1366 preference votes, or less than 10 per cent of the formal votes cast in the poll.

The court heard staff at the Electoral Commission became "suspicious" after an "unusually large number" of online enrolment applications for voters in the Auburn election were submitted just before the electoral roll closed in July 2012.

Analysis of Ms Mehajer's computer showed "saved images of receipt numbers" for some of these applications, he said.

The forms were submitted from IP addresses associated with Mr Mehajer and his family.

Mr Rapke said 76 false applications were submitted, 51 of which were forgeries where the person who purportedly signed the form had no knowledge of it.

He said text messages between Mr Mehajer and his sister revealed their plan to submit false enrolment forms to bolster their chance of winning the election.

The court heard the siblings set up two groups to contest the election in each of Auburn Council's two wards. Mr Mehajer was the lead candidate in one group and his sister in the other.

Mr Rapke said running for election as part of a group increased a candidate's chances of getting elected. A number of the candidates in the siblings' groups were not eligible for election, the court was told.

Ms Mehajer did not plead guilty to offences in relation to the forged application forms but did plead guilty to providing false or misleading information to the Electoral Commission. She will return to court on June 26.

The trial continues.