Flood flows on the Buller River this month were the largest of any New Zealand river in almost 100 years, Niwa measurements show.

Flow rates on the river during the flooding on the West Coast earlier this month were more than 10 times the usual rates, scientists found.

Niwa hydrodynamics scientist Richard Measures said records showed even greater flooding of the Buller River in 1926.

"It's not the largest river in New Zealand but it does experience the biggest flood flows," he said. "The flood levels through the Buller Gorge are incredible."

Niwa environmental monitoring technician Mike O'Driscoll used a radar gun from the Westport and Orowaiti bridges to measure the speed of the Buller River at its surface on July 17, the height of the recent floods.

Those measurements, coupled with surveys of the riverbed level carried out from a jet boat after the flood, provided the data for flood-flow calculations.

As the bridges were being closed, the measurements showed a flow of 7640 cubic metres per second - the largest direct measurement of river flow ever completed in New Zealand.

The previous record was a flow measurement of 5870 cubic metres per second taken in the Grey River in 1988. As a comparison, the mean flow of the Buller River is 454 cubic metres per second.

A Niwa monitoring station on the Buller River at Te Kuha, about 10km upstream, was continuously recording water levels throughout the flood.

The monitoring station is part of Niwa's national network of water-flow monitoring stations, as well as being a key flood-forecasting site for the West Coast Regional Council.

The site has three independent monitoring systems in place - a sensor system to provide rapid updates during floods, plus primary and back-up sensors to record data for future analysis.

But the magnitude of the flood meant debris knocked two sensors out of action, leaving only the back-up providing real time information to inform emergency management decisions.

The water level at Te Kuha peaked at 12.8 metres at 1.35pm on July 17, several hours ahead of the peak flooding in Westport.

In the 1970 flood the flood peak was 11.9m. This was the biggest flood recorded since the Te Kuha monitoring site was established in 1963.

"We can confidently say that the recent flood flows in the Buller River are the highest of any river in New Zealand since 1926, when historic records and photographs of flooding show an even bigger flood on the Buller," Measures said.

Locals view the rising water at the Buller bridge in Westport on July 17.
Locals view the rising water at the Buller bridge in Westport on July 17.
(Read more here)