The Mataura River is the major river responsible for the presence of the Awarua Plain; along with the Oreti River, it is a remnant of the rivers from the Ice Ages that formed the Southland Plains (Murihiku).
When this name was given to the then new railway station, residents of Castlerock, then also known as The Elbow and on the opposite bank of the Oreti River, brought the matter to George Lumsden of the Otago Provincial Council.
It was described in 1893 by New Zealand naturalist, ethnologist and museum director Augustus Hamilton, from material he had collected the previous year at Castle Rocks on the Oreti River in Southland.
Twenty-six kilometres in length, the bay lies between the town of Riverton and the outflow of the Aparima River in the northwest, and the estuary of the Oreti River in the southeast.
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About 78% of the population breeds in the Southland Region on the southern end of South Island, New Zealand, especially beside the Mataura, the Oreti, the Aparima and Waiau Rivers.
After ending concurrency and crossing the Oreti and Aparima Rivers the highway continues proceeding in a north-westerly direction towards Wreys Bush and Nightcaps.