Wanganui District Council chief executive Kevin Ross said when the Whanganui River flooded it was usually caused by large volumes of water upriver from Pipiriki.
"However, this time we have had a large inflow of water from Pipiriki to Wanganui, and that is unusual for us," Mr Ross said.
Evacuation centres had been opened at St Paul's Church in Guyton St and at Girls College in Jones St, Wanganui East, and around 200 people had been processed by the evacuation centres.
"We have put 69 people in motels, with the rest staying with family or friends," Mr Ross said.
The river levels began to drop yesterday morning, and Mr Ross said road crews will soon begin to clear the streets of water, silt and debris.
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He asked people who had been evacuated to stay away from their homes, as many will need to be assessed by geotechnical engineers. This process is due to begin today. "These properties will then be cleaned and limed due to sewage contamination."
Evacuated areas in Taupo Quay, Anzac Parade and other flood and slip affected areas were cordoned off, and monitored by police and army personnel. Mobile police patrols were on duty throughout the community. Mr Ross said tap water is safe to drink, however it was essential people conserved water.
"The water supply is minimal due to power outages. Please don't use hoses or large amounts of water to clear driveways or streets from flood waters."
Bernie Rush from Wanganui Fire Service said firefighters from Wellington had been brought up to assist local crews. One fire crew was based in Wanganui East.
John Stretton from St John said Saturday had been a reasonably quiet night, although there had been a few emergencies such as a Marton woman giving birth in the back of an ambulance.