Special-purpose authorities differ from territorial authorities in that each is charged with only one major function. The need for the most efficient and economic discharge of the major function being the prime consideration, their boundaries may either extend beyond or fall within those of territorial authorities in the same geographical area. Only rarely do the boundaries coincide. Sometimes, as is the case with a number of urban fire authorities, territorial authorities themselves are also constituted as and perform the functions of special-purpose authorities. The more numerous and important special-purpose authorities are those administering harbours (32 harbour boards), the retail distribution of electricity (43 electric power boards), and rabbit and opossum destruction (180 rabbit boards). Others are engaged in milk distribution, fire fighting, water supply, urban drainage and transport, soil conservation and rivers control, nassella tussock control, land drainage, and underground water usage. The Auckland harbour bridge and the Lyttelton road tunnel are also controlled by special-purpose authorities.