The Otago Harbour Board may from time to time make by-laws providing that harbour-improvement rates not exceeding in any case five shillings per ton, reckoned either by weight or measurement as shall be expressed in such by-laws, shall be levied upon all goods and merchandise discharged at or shipped from the Port of Dunedin, and the said Board may from time to time repeal, alter, or vary such by-laws, but so that the maximum rate to be imposed shall not exceed the sum of five shillings per ton as hereinbefore provided.