A Sydney councillor has one final chance to apologise for a tirade against a fellow councillor, whom he described as a "Judas", before being reported to the state government.

Blacktown councillor Jess Diaz made the offending remarks two months ago after mayoral elections. He did not attend a council meeting on Wednesday night to deliver a required apology after the speech was found to have breached the council's code of conduct, leaving him a final chance in the new year.

Cr Diaz, the patriarch of a powerful Liberal Party family whose son Jaymes​ failed in a federal electoral bid after being embarrassed in a media interview, has long sought to be Blacktown's mayor.

He missed that prize for a second year in a row, and by just one vote, after his former ally Jacqueline Donaldson resigned from the party shortly before the vote. Cr Donaldson's change of vote handed the mayoralty to an ALP councillor.