Wash. Rev. Code § 41.50.125
The department may charge interest, as determined by the director, on member or employer contributions owing to any of the retirement systems listed in RCW 41.50.030. The department's authority to charge interest shall extend to all optional and mandatory billings for contributions where member or employer contributions are paid other than immediately after service is rendered. Except as explicitly limited by statute, the director may delay the imposition of interest charges on late contributions under this section if the delay is necessary to implement required changes in the department's accounting and information systems.
RCW 41.50.125
Findings-1994 c 177: "The legislature finds that:
(1) Whenever employer or member contributions are not made at the time service is rendered, the state retirement system trust funds lose investment income which is a major source of pension funding. The department of retirement systems has broad authority to charge interest to compensate for the loss to the trust funds, subject only to explicit statutory provisions to the contrary.
(2) The inherent authority of the department to recover all overpayments and unauthorized payments from the retirement trust funds, for the benefit of members and taxpayers, should be established clearly in statute." [ 1994 c 177 s 1.]