"There will be extremely large rainfall amounts for some [areas]," he said.
Mr Coutts said the most significant rain and wind would be in Whanganui and Taranaki. At Mt Taranaki, up to 400mm of rain was forecast which could affect the Whanganui River.
NIWA forecaster Ben Noll said the conditions were caused by the remnants of tropical cyclone Debbie. Bringing winds of more than 260km/h, this damaging cyclone blew through Queensland last week, causing several fatalities and widespread destruction.
A Whanganui District Council spokeswoman asked people to check their emergency supplies and make sure their guttering and drains were clear of debris. Farmers are also warned to move stock away from riverbanks.
The spokeswoman said emergency management staff were watching the weather and keeping a close eye on the Whanganui River and other rivers and tributaries.
"We have contacted all emergency operations centre staff to check their availability if needed," the spokeswoman said.
Horizons Regional Council confirmed that its emergency coordination centre was opened this afternoon. Horizons' staff would be keeping a close eye on river levels.
Horizons controller Ged Shirley said staff would also be in regular contact with locals councils.
"There is the potential that the Moutoa and Makino floodgates and Whanganui flood barriers will be activated Wednesday afternoon. We have operators on standby if required and we will notify farmers who use the spillway land as soon as possible," Mr Shirley said.
If the expected rainfall arrives, it will be far greater than the 140mm which fell in Whanganui on June 19 and 20, 2015, causing widespread flooding throughout Whanganui, Rangitikei and South Taranaki.
