The Maitai/Mahitahi/Maitahi River is the largest in the Nelson Region with a catchment of over 9,000 ha. The river rises in the Bryant Range behind Nelson City and the upper catchment has two branches draining conservation and water supply protection land. The North Branch is dammed just upstream of the confluence with the South Branch to form the main Nelson water supply storage reservoir.
The mid catchment is an important recreational and production forest area, and the lower catchment runs through the heart of Nelson City, before flowing into The Haven.
Two large tributaries are also monitored as part of the catchment; Sharlands Creek and Brook Stream.