Mr McDouall said the state of emergency had been declared after he'd received information from Horizons Regional Council hydrologists that Whanganui could be faced with a "significant" flood.
"We're lucky that we have some time - we didn't have time in 2015."
People in the affected areas should begin preparations immediately, he said.
Pets should be ready to leave home if necessary, vehicles should be out of the way of flood waters and important possessions should be moved.
People who evacuate their home should check in with a Civil Defence Centre, either arriving in person, or by calling Whanganui District Council on 06 349 0001. People are reminded to take necessary medications with them and advise officials of any chronic or acute health needs.
Two Civil Defence centres are open at Whanganui Girls College in Jones Street, Whanganui East, and St Paul's Church, opposite the Whanganui District Council building in Guyton Street.
"Sadly, I have to say that the comment from Horizons was that the flood may reach historic levels," Mr McDouall said.
Rangitikei District Council has also declared a state of emergency.
All schools in the Whanganui and Rangitikei districts will be closed today - an automatic move whenever a state of civil emergency is declared.
A Ministry of Education spokeswoman said they were advising all early childhood education services in the districts to also close.
"We are contacting all schools and early childhood education services in the Whanganui and Rangitikei districts to ensure they are aware of the need to close," she said.
If parents are unsure if their school or ECE is affected, they should visit the school or ECE website or Facebook page or contact the school or ECE directly.
Schools will remain closed until the state of emergency is lifted.
Rainfall was steady in Whanganui yesterday, with 41.6 millimetres of rain falling up to 6pm last night. The Whanganui River reached an initial peak of 8.7 metres at Pipiriki around 3pm.
Last night Horizons issued a flood warning for the Whanganui, Whangaehu and Turakina rivers.
Horizons controller Ged Shirley said yesterday's rainfall, combined with an updated Metservice forecast of 150-250 millimetres of rain for the next 36 hours, had impacted previous river level predictions.
"The Whangaehu is expected to peak at 15.3 metres at 3pm, which is 4m higher than 2004, and is likely to close State Highway 3 at approximately 11am. The Turakina is expected to peak at 11.6m at 6.45pm.
"Horizons are currently calling landowners in the Whangaehu and Turakina areas to advise moving stock and possessions. We are also contacting landowners who use the Moutoa and Makino spillways as floodgate operations are likely tomorrow morning."
