This is my DIY First Flush diverter system. Used to remove the dirty roof water that heads towards your tank when it begins to rain.

You will need
90mm Tee Joint
90mm Pipe
90mm-100mm reducer
100mm pipe
2x 100mm coupling (1 must be threaded)
100mm Threaded Cap
100mm Styrofoam ball or Water Bottle slightly narrower than 100mm

Facebook:   / carazypermaculture  
Website & Instructions: http://carazy.net/how-to-build-a-rain...

Have an awesome day!

** UPDATE 29 Mar 2015 **
It's now Autumn and we've gone through 7 months with some massive tropical storms and it's working a treat. I've drilled the hole out to 6mm as it wasn't draining as well as I'd like. I've only cleaned it out 3 times and each time there's barely been half a cm (quarter inch) of gunk in the bottom. Today there were 3 dead geckos and a small amount of dirt so it's working well.

*** Addressing some of the confusion as to what this system does:
First flush diverters are NOT designed to remove large debris like leaves etc. Their purpose is to remove the fine dust and sediment that builds up on your roof and if it enters your tank you get sludge/mud buildup at the bottom that is difficult to remove. First flush diverters minimise this. You should absolutely be addressing the issue of leaves and twigs at the gutter level BEFORE they enter your downpipes, not just before it enters the tank or have a chance to clog in your pipework. I do this with one of the many forms of gutterguard (see link above). Alternatively you can put some sort of non-fine mesh where the water exits the gutter. I hope this clarifies the objectives of these devices.