Mararoa River is in the South Island

South Island

The South Island is the larger of the two major islands of New Zealand, the other being the more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman Sea, to the south and east by the Pacific Ocean...

 of New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

. The river feeds into the Mavora Lakes

Mavora Lakes

The Mavora Lakes are located in the South Island of New Zealand.They consist of two lakes named North Mavora and South Mavora. The area has rudimentary camping sites and is maintained by the Department of Conservation. This park was also used as a film location for the Lord of the Rings movie...

 and discharges into the Waiau River

Waiau River, Southland

Waiau River is the largest river in the Southland Region of New Zealand. It is the outflow of Lake Te Anau, flowing from it into Lake Manapouri 10 kilometres to the south, and from there flows south for 70 kilometres before reaching the Foveaux Strait eight kilometres south of Tuatapere...

 eight kilometres downstream from where it leaves Lake Manapouri

Lake Manapouri

Lake Manapouri is a lake in the South Island of New Zealand. Its name is Maori for "sorrowful heart", though this name is misapplied due to an early cartographical error...

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Water from the Mararoa River is feed up the Waiau River by a control structure at the confluence of the two rivers. This is used to control the level of Lake Manapouri for hydroelectric power generation.

Didymo, an invasive organism that was discovered in New Zealand in 2004, has been found in the river.