California Family Code
Family Code section 3552
(a) In a proceeding involving child, family, or spousal support, no party to the proceeding may refuse to submit copies of the party's state and federal income tax returns to the court, whether individual or joint.
(b) The tax returns may be examined by the other party and are discoverable by the other party. A party also may be examined by the other party as to the contents of a tax return submitted pursuant to this section.
(c) If the court finds that it is relevant to the case to retain the tax return, the tax return shall be sealed and maintained as a confidential record of the court. If the court finds that the tax return is not relevant to disposition of the case, all copies of the tax return shall be returned to the party who submitted it.