Floodgates were opened early on Wednesday morning after heavy rainfall overnight resulted in a dramatic rise of water levels in the Manawatū River.
At 8am the water had risen by eight feet at the spillway.
By midday the flood water had reached the temporary bridge over the Manawatū River at Foxton, flooding the work site below the Whirokino Trestle next to it.
Plenty of debris came down with the flood waters, including logs, clogging the river flow at the bridge.
Horizons Regional Council's group manager river management Ramon Strong said on Wednesday morning that the Manawatū River peaked at 5.1m at the old Teachers' College recorder site.
He said he suspected a sustained peak, which meant the gates were left open.
Council staff monitored the flood as it worked its way down the main stem of the Manawatū and had crews at the gates.
Strong said as water levels subsided his staff would begin inspections of river systems and flood defences to assess any damage.