Hot tap in kitchen turned on = Hot shower upstairs goes cold

[SydneySteve]

As the title says, when the hot tap in the kitchen is turned on, the hot shower upstairs goes cold.

The house is about 15 years old.
I have an electric hot water heater.
The kitchen sink has two taps (hot and cold) plus faucet.
Ditto the shower (except shower head not faucet).
All taps are old fashioned turning type (yes I know I'm using all the industry jargon here! haha).

I have also noticed that the cold tap in the shower does not need to be turned very much for maximum water pressure to be achieved, whereas the hot tap needs turning much more.
A small turn of the cold tap changes the temperature from the shower head quite dramatically - there is a sweet spot of just a few degrees turning (maybe 10 to 20 degrees, on a circle, not temperature) where you get a reasonable temperature (too far either way and you will burn or go cold).

What is the problem and solution?

 

[JohnD]

Australian plumbing is different from UK plumbing.

Have you got a cold water storage tank?

 

[SydneySteve]

No. There's no cold water storage tank.

 

[JohnD]

if the pressure and flow in the hot/cold taps, and the hot/cold supply are balanced, them it is probably a fault in the mixer.

however, if the flow from the hot tap is weaker than the cold, you may have scale or other obstructions in your water heater or a valve.

This is a UK website, you may do better with someone nearer home. The plumbing really is very different.