The fact that you have very high Nitrites might be an indication that your biological filter can not handle the waste load. Did you medicate, causing your Nitrobacter to die off? Maybe you are cleaning your filters too often? Might also be caused by your source water, ie, bore hole. Do you have sludge buildup somewhere in your system?
After the heavy rains in Gauteng, a lot of people using borehole water experience problems with high ammonia and Nitrites. The rain contaminated the surface water in some areas.
The only real effective way to get rid of it quickly is to do water changes provided that your source water is fine. In your case, you should probably do large changes of up to 30% at a time.
Nitrite enters the gills and impairs the red blood cells ability to transport oxygen through the koi's blood supply, eventually leading to nitrite toxicity. Salt effectively reduces the toxicity of nitrite. I suggest that you add up to 3kg/1000 pool salt to your pond after a 30% water change.
Stop feeding until your Nitrites are at acceptable levels.