PRESSURE KEEPS RISING WATER KEEPS POURING OUT THE OVER FLOW?

THE PRESSURE GUAGE RISES TO 3BAR AND WATER COMES OUT THE OVERFLOW CONTINUALY ONLY STOPS WHEN I TURN OFF THE COLD WATER AT THE MAINS.THE BOILER IS NOT ON.
I ALSO HAVE A LEAK IN THE HOT WATER TANK AND I AM WAITING FOR IT TO BE REPLACED IS THIS JUST A COINSIDENCE OR ARE THE 2 RELATED ? IT IS A PRESSURISED SYSTEM .