We received email from builder as below about the overflowing water for our water tank:
"This is due to the filter bag with in the tank being built up with silt and other debris. It requires frequent cleaning. I have recommended to some clients to remove filter altogether because of this problem. However this is entirely your decision.
Alternatively, you can contact Bluescope Steel regarding other remedies. You will find the number on the tank."
As I got no idea what he is talking about, I asked how to locate this thing. Then received further details from builder:
"Looking at the pictures you sent me..... The large black plastic rings the water flows into. The first one is the leaf strainer (coarse filter) The second one - directly underneath is the filter sock... when you remove this you will understand why it is called a filter sock.
You can view all the rlevant information on Bluescope website. you can obtain this with a simple google search under bluescope water."
Not quite sure what this "filter bag" is, did a quick search and found this on Google from Rain Water Tank Direct web site.
Will try to have a look during the weekend...