1. Prefix added to verbs which express some kind of action to form nouns denoting a human agent (i.e. the person doing the action), e.g. kaikōrero (speaker), kaitiaki (guardian, trustee), kaimahi (worker), kaihoko (seller), kaituhi (writer) and kaitito haka (haka composer). This prefix cannot be used with verbs the meaning of which indicates a state, not an activity, e.g. moe and tū. In other words, only transitive verbs can take the prefix kai-. There are exceptions to this rule, notably kainoho (inhabitant). Some of the words created by this use of kai- are listed in this dictionary, but the list is not exhaustive.
(Te Pihinga Textbook (Ed. 2): 48-49;)