Seventy homes in Nelson are being evacuated due to Maitai River rising rapidly as heavy rain continues to sweep across the South Island.

Earlier this afternoon, a voluntary evacuation order was announced for parts of Westport, Seddonville and Mokihinui.

At a media conference, Buller Emergency Management Controller Al Lawn and Buller Deputy Mayor Sharon Roche says it's a precautionary measure.

"We would rather take a precautionary approach ... rather than knock on doors at 2 or 3 in the morning," Lawn says.

Roche says the West Coast was well prepared after several flooding incidents in recent months.

"This is not unusual or new to us."

About 140 households in three areas were being asked to voluntarily evacuate. See here for complete details.

Lawn says although the weather event hsd been slightly downgraded from what was expected, they didn't make the decision to evacuate certain areas lightly.

"It's easier to do it in daylight. The abundance of caution is there.

"Hopefully nothing happens. I really hope no houses get flooded. No people come home and go 'oh no, not again'."

Meanwhile, further north in Golden Bay and Nelson, multiple roads are closed and some people are also voluntarily evacuating.

An orange heavy rain warning is also in place for Auckland and Northland.

Westland Civil Defence says residents should be ready to evacuate if required as weather conditions worsen. A state of emergency is in place across the West Coast as forecast rain hits.

The area around Franz Josef has seen the heaviest rain, as the West Coast and top of the South Island start to feel the bad weather.

Hydrologists have stayed overnight at the nearby Waiho River to monitor rising levels.

Civil Defence urged Westport residents whose properties had previously flooded to keep a bag packed as they may have to evacuate their homes.

A state of emergency was declared after severe weather warnings were raised to red for Buller and Westland until Thursday afternoon.