It has two dialects, called Austronesian and Papuan. Both dialects are of course Austronesian in both grammar and vocabulary due to their original derivation − the dialect names refer to the "first languages" spoken by users of this lingua franca. The Papuan dialect (also called "Non-central") was in the language's heyday much more widely spoken, and was used as the standard for official publications, but the Austronesian (or "Central") dialect is closer to Motu in grammar and phonology, and its vocabulary is both more extensive, and also closer to the "original" language. It tended, for this reason, to have a much higher status, and was regarded by almost all speakers as more "correct".
The language has a history long pre-dating European contact; it was originally used by participants in the Hiri trade cycle (principally in sago and clay pots) between the Motu people and their neighbours on the south east coast of the island of New Guinea. In early colonial days its use was spread by its adoption by the Royal Papua Constabulary (hence the name "Police Motu"). Tok Pisin was not widely used in Papua New Guinea south of the Owen Stanley Ranges until after World War II, and by the early 60s Hiri Motu had probably reached its widest use, being the normal lingua franca of a large part of the country. It was in fact the first language of many people whose parents came from different language groups. However, since the early seventies, if not earlier, the use of Hiri Motu as a day-to-day lingua franca in its old "range" has been gradually declining in favour of Tok Pisin; speakers nowadays tend to be elderly, and concentrated in Central Province and Gulf Province. Reflecting this situation, younger speakers of the "parent language" (Motu proper) tend to be unfamiliar with Hiri Motu, and few of them understand or speak it well, which was certainly not the case a generation or two ago.