Watch: Waiho River Bridge destroyed by surging storm in South Island

The storm tearing through the Westland has destroyed the Waiho River Bridge, south of the Franz Josef township.

Rivers in the South Island are raging out of control as rain and heavy winds continue to batter the region.

Most rivers in the region have hit alarm level, and over 300 kilometres of highway has been closed due to slips and flooding.

Eleven people who were trapped by the flooding have been rescued from a track north of Lake Whatipu.

It is unclear whether two hunters who were in trouble south of Fox Glacier have been rescued, according to the West Regional Council rescue coordination centre.

An "atmospheric river" of pressure left over from Australia's cyclones hit the South Island on Monday, and have been causing an "exceptional" amount of rain since.

The western ranges are expected to receive half a metre of rain. 

"For the West Coast it's going to produce exceptional amounts of rainfall in the ranges but unusually also on the coast as we've seen with Milford sound, and a significant spill-over event for eastern parts of the divide," said meteorologist Georgina Griffiths.

"It's been 20 years since we've seen the rain levels that we're currently seeing at Milford sound, 430mm in the gauge already recorded there."