Hidey ho,

So I was working in the garage over the weekend and whilst in there noticed water spewing over the edge of the roof on the house. It wasn't raining, so like any good kiwi I went into seek and destroy handyman mode.
My eyes advanced upward and saw the source of the sorcery - the HWC head pipe/overflow pipe.

I beckoned to the wife, and it turns out she was cleaning the upstairs shower, but mostly on cold water.

So our HWC is on the ground level, and the shower in question is on the first floor. This shower does suffer from low pressure on hot water. 
I don't believe the HWC is mains pressure - probably medium pressure as I don't believe we have a header tank. 

My question is - would one expect water to come out the header pipe when the shower mixer on the upstairs shower is set to anything but full hot water?