Nitrate is NOT as directly toxic as ammonia and nitrite. Higher levels are an added stressor and believed to stunt the growth of koi. Symptoms include overall lethargy, koi that never seem to grow and koi with stressed immune systems opening them up to a wide range of bacterial infections.
Once you start showing nitrates in a new system, you know your biofilter is functioning. To what degree, depends on your other parameters. You may still be showing ammonia or nitrite, but the cycle has begun (or completed --- depending on how you look at it). Plants and algae use nitrate as a food source. Most ponds will have a nice coat of algae on the walls and bottom. This usually keeps most ponds in the safe zone. 10% water changes every 1-2 weeks will further reduce nitrate levels --- it's good for overall pond health also. If readings go above 40 ppm, a greater water change is recommended.
Some of the more common REASONS for an elevated nitrate level are :
1 - infrequent water changes
2 - overstocked
1 - more frequent water changes
2 - get rid of some fish or build a larger pond