Eyewitness describes 'unbelievable' moment West Coast bridge collapsed

An eyewitness to the Waiho River bridge collapse says it was a "once-in-a-lifetime" experience.

The West Coast bridge collapsed in front of onlookers on Tuesday after heavy rain across the region made river currents torrential.

Jacob Schonberger filmed the destruction - something he describes as unbelievable. 

"It was just something you never see ever in your life, it's a once in a lifetime thing really, just amazing, unbelievable."

Schonberger said the bridge had looked sturdy but, as it began to collapse and the road tore free of the piles, it made a lot of noise which may not have been picked up in the video of the collapse.

"[There was] lots of banging and twisting of steel...  sounding similar to a ship docking into a harbour or something.

"Lots of loud, thunderous groaning of steel and metal."

He said the bridge began to fall apart when one of the riverbanks began to wash away.

Schonberger said his phone went "ballistic" as the video went viral.

A state of emergency was declared for the region on Tuesday.

Further rain fell overnight and Civil Defence said in a statement on Wednesday morning there is widespread flooding in Westland.