If the cistern is a proper potable water cistern, it should have a built-in manway or hatchway that is large enough for an average sized person to enter. Therefore, a person can enter the cistern and scrub the ceiling, walls, and floor with warm water and dish soap solution. I have also used a small amount of liquid household bleach, following the instructions recommended for cleaning kitchen countertops and sinks, etc. If the cistern hatch is too small to allow a person to enter, clean of the cistern can be done through the hatch with aids such as telescopic poles for painting, etc. Perhaps even a power washer wand could be inserted through the hatch and utilized, offering the advantage of water pressure. In the past, I have cut an opening in the top of a water cistern large enough to allow easy cleaning. I then fabricated a cover for the opening out of plywood, rubber gasket material, and stainless steel bolts. The piece cut out of the cistern can be fastened to the plywood cover so to fit back into the opening where it was cut from. If a person is to enter the cistern through an appropriate hatch, it is very important to be certain the cistern is well ventilated before and during the cleaning process.