hoffy. Sounds like you are from Australia from your description.
In Australia “Queensland” the AS3500 calls for a max pressure of 500kp, the recommendation is to install a 500kpa PLV at the meter, however a lot of plumbers install the plv at the HWS.
Some councils, because of TDS or calcium levels in the supply require a HT50 or PRV to be installed — so if the HT55 valve (high temp & high press reliefe valve) is set to 1000 kpa the install a 850kpa prv.
If you have replaced this item in Australia, you have carried out illegal plumbing! & I cannot in all honesty abet any illegal work. & you have by your DIY attitude voided your warranty on the HWS.
Call a Licensed Plumber.