N.J. Stat. § 58:5-23
After the actual completion of the plant and the commencement of the actual supply of water therefrom, the expenses of the commission and its agents and employees shall be charged to operation and properly proportioned as between the several water supply projects which may be operated by the commission.
After the plant is completed or acquired the cost of operation shall be estimated yearly in advance and apportioned among the municipalities respectively in proportion to the amounts of water contracted to be used by each, and each of the municipalities shall pay said sums to the commission in advance when requested.
At the end of each year the commission shall make an apportionment of the actual cost of the operation of the plant among the contracting municipalities upon the basis of the actual water consumed by each municipality, but such amount shall be in no event less than the quantity contracted for. In apportioning said cost, no municipality shall be charged with any item of interest or rental upon, or cost of operation of, any part of any water plant which is not used in supplying water to the municipality, but any municipality may contract to bear the proportionate cost of acquisition and operation of any water plant which may be devoted in whole or in part to the service of one or more municipalities. Each municipality shall be charged with the amounts so apportioned, and credited with the amount previously paid on account of the estimated operating expenses for such year.
N.J.S. § 58:5-23