Hundreds were evacuated, cars were "tossed" around car parks and water swept through buildings after a West Coast river breached its banks overnight.

The Waiho river, near Franz Josef, flooded north of the town at 11.50pm on Wednesday as heavy rain fell.

 The river flowed quickly through nearby buildings and cars were thrown around car parks. State Highway 6 closed. A local state of emergency was declared at 2am.

Westland Civil Defence public information manager Andy Thompson said the flooding had disrupted the town's water supply. Engineers would assess the issues, but residents should conserve water and boil the tap water if they had concerns.

Tourists evacuated, state of emergency, after West Coast river breaks banks | Stuff.co.nz