catchment tank overflowing the top

[MarkP]

Does your overflow pipe hook up and over the rim of the tank? If so then it is the type that works by siphoning. It would have a U inside the tank and one outside the tank plus the upside down U that goes over the rim of the tank. It has to be solid full of water to work. Force it under inside the tank and turn it every which way to get the air out. If on the other hand you have a penetration through the side of the tank then it should flow naturally. If not you have a blockage.