the relationship between those items is highly effective on an ongoing basis.
The meaning of this sentence is: "The relationship between those items is highly effective."
The contribution made by on an ongoing basis is minimal or non-existent, but it is a phrase popular with those who like to throw extra words in "just for fun." It falls into the same category as "at this point in time," which actually means "now" and is usually totally superfluous anyway.
If you really want a definition for "on an ongoing basis," then perhaps "continuously" fits the bill. Or "now and in the future." But the verb in your sentence is "is," which in no way implies that the effectiveness of the relationship is about to come to an abrupt end. The continuous nature of the effectiveness would be assumed anyway.