It shall be the duty of such board of trustees to supervise the maintenance of such sewer system constructed under its supervision, or as may hereafter under proper authority be placed under its supervision, and to see that sewer system or systems be maintained in good condition; provided, however, that such board of trustees may, whenever in the opinion of said board of trustees it may become necessary, employ a person or persons, firm or corporation, of its selection, to do and perform such repair services as may be necessary to maintain said sewer system or systems in good condition, and may, if in the judgment of the board of trustees the occasion demands, employ an engineer to plan and supervise any such repairs; however, such board of trustees shall not have authority to expend within one year for maintenance purposes, including expenses of said district, more than the amount of the maintenance tax provided for in section 249.130.