eho your installation is very similar to ours (apart from size and shape of tank). You have a "charged system" in which all the downpipes are linked together underground with sealed joints, and they contain water up to the level of the inverted "U" above your tank. As soon as water flows off the roof, the charged system overflows from the "U" into the tank - powered by gravity as you thought .
If that is not your electricity box beside the tank, it will be the "Rain Bank" valve that controls the supply to your green plumbing - toilet cisterns, washing machine tap and a garden tap. You probably also have your tank water pump submerged inside the tank like ours. Even if you can't get into your house, you might find the manuals for the Rain Bank and the pump hidden in that wall box.
<edit> Here's a link to the Davey page for RainBank - you can get a PDF version of the RainBank manual:
http://www.daveypumps.com.au/products/product_view.aspx?id=245&catID=242&app=Household+Water+Supply
I see your power point is above the box - ours is inside it.