New Zealand Prime Minister Jacinda Ardern Thursday welcomed Facebook's move to ban white nationalism on its platforms, but she said more must be done to prevent slaughtering like the Christchurch mosque attacks.

Severe flooding forced thousands of residents in New Zealand's South Island to flee their homes on Wednesday and left hundreds of tourists stranded at the remote Milford Sound beauty spot.

The Southland region declared a state of emergency after being deluged with more than 1,000 millimeters of rainfall in 60 hours, triggering landslides on major roads and causing rivers to burst their banks.

Authorities told residents in the low-lying areas of Gore and Mataura to evacuate immediately early on Wednesday as floodwaters in the Mataura river peaked, warning those further downstream in Wyndham to prepare to leave.

Residents were advised to grab medication, clothing and identification documents, then head to higher ground.

Power to affected areas was cut off as a precaution and evacuation centers were set up in local churches and schools.

Floodwaters washed away sections of the only road to Milford Sound, a popular hiking spot for international tourists, and EMS said almost 200 people were being airlifted to nearby Te Anau.

Only two minor injuries have been reported after a landslide hit a hut on the Routeburn walking track.

AFP