California Code, Family Code - FAM § 3629

(a) At the hearing on the application for the expedited support order, all parties who are parents of the child or children who are the subject of the action shall produce copies of their most recently filed federal and state income tax returns.

(b) A tax return so submitted may be reviewed by the other parties, and a party also may be examined by the other parties as to the contents of the return.

(c) Except as provided in subdivision (d), a party who fails to submit documents to the court as required by this chapter shall not be granted the relief that the party has requested.

(d) The court may grant the requested relief if the party submits a declaration under penalty of perjury that (1) no such document exists, or (2) in the case of a tax return, it cannot be produced, but a copy has been requested from the Internal Revenue Service or Franchise Tax Board.