The Supreme Court action was brought by the NSW Electoral Commission after the iVote system "crashed" during council elections in December.

In his ruling, NSW Supreme Court Justice Robert Beech-Jones said the failure of the iVote system during last year's local government elections on Saturday, December 4, meant that each election was not "conducted in accordance with the General Regulation" as was required by section 310 of the Local Government Act.

"Even though the number of voters denied the franchise by the failure of the iVote system was small, the system of proportional representation utilised in local government elections is such that the votes that were denied to those voters had the real potential to affect the election of at least one Councillor in each of the three subject elections," Justice Beech-Jones said.

"Once that conclusion is reached then the scope of the Court's discretion to refuse to declare the election void in whole or in part is limited.

"With considerable reluctance I consider that, because the system of election for the three Councils is proportional representation, it is necessary to declare all of the Councillors' elections void."

The court case examined results for Kempsey Shire Council and one of four wards in Shellharbour City Council as well as for councillors in Singleton.

The ruling does not affect the outcome of the mayoral race, which was won by incumbent Mayor Sue Moore.

In a statement, Singleton Council general manager Jason Linnane said the council was aware of the decision handed down by the Supreme Court.

"At this point in time, Council has not been briefed by our legal team and cannot make an informed response until that occurs," Mr Linnane said.

"However, it is apparent we will be undertaking a re-election of the nine councillors. When that will occur will be determined by the court at the next hearing on 5 April 2022."

In his ruling, Justice Beech-Jones stated it was necessary to emphasise that nothing in these proceedings involves any "alleged failing much less misconduct on the part of the Councils or any of the candidates, including those who were elected".

"To the contrary, the Councils and the candidates, including the elected Councillors, are completely innocent of any of the conduct said to have caused the elections to miscarry," Justice Beech-Jones said in his ruling.

"They will unfairly bear the consequences of the failure of the iVote system."

The court ordered that proceedings stand over until 9:30am on Tuesday, April 5, and Justice Beech-Jones said at that time he would hear submissions about the forms of order necessary to give effect to the court's judgment.