Why is the water coming from my overflow outlet pipe under extreme pressure?

I assume that you're referring to a water heater T&P relief valve, Correct? With no thing to go on it's going to be hard to answer your question.
1) is it a water heater that's the problem?
2) gas or electric?
3) is the blow off water boiling hot?
4) "Extreme Pressure"! You mean it's higher then house pressure? How much higher?
Back to you, Tom