“Manawatū has its origins in a waiata, Ka tatū e hine ko Manawatū, describing the search by ancestor, Haunui-a-Nanaia, for his wife.

He named the waterways on his quest and the waiata talks about how his heart (manawa) settled or momentarily stopped (tatū) when he saw the river. Manawatū is an amalgamation of those two words.”

Poetically beautiful.

From the RNZ article Seal of approval for spelling of Manawatū-Whanganui region.