Another way to increase the downpipe capacity, is to increase the throat diameter by having a conical entrance
to the downpipe. Hence increasing the length of the entry weir.
However you should approach a consultant on this as it is not always applicable.
A down pipe designed to the code does not flow anywhere near full, it is the entry orifice/weir that limits the flow. If you want to investigate this further, check out the notes on the pipe size calculator Charged downpipes. and pipes flowing to tanks, or discharging under water.
A "charged" downpipe is one that flows full, or stays full because of a "U" shape.
I would also be interested in any modifications, or suggestions that you would
If you need more info, or you would like other areas of Australia, or New Zealand, added to the list,
please send me an email. (firstname.lastname@example.org)