Some people in the Bay of Plenty town of Edgecumbe will never be able to go back to their homes, the Prime Minister says.
The Rangitāiki River burst a bank yesterday and inundated the town, sparking a mass exodus.
Almost the entire town, 1600 people, was evacuated and a state of emergency was declared.
Mr English said he had visited the town and the Fonterra plant and there would be "significant costs" for homeowners and farmers.
"There's 900 empty households, and some proportion of them... no-one will ever go back.
"We want [residents] to know that they are fully supported, we will do whatever we can to give them some certainty.
"And we will stick with them because we know that for some of them this is going to be a longer process."
An investigation would be needed to determine how the breach occurred, he said.
Social Development Minister and local MP Anne Tolley said the ministry was working with the emergency team to put together a support package.
"But it is very clear that we are going to have to support a significant number for quite some time."
Whakatāne district mayor Tony Bonne said work on blocking holes in the Rangitaiki River's stopbanks in Edgecumbe would go ahead this afternoon if the floodwaters kept dropping.
Pumps from all over the country would be used to extract the water, which could take up to two weeks to clear, he said.
Much of the Rangitaiki Plains was below sea level, and rural areas were still at risk as the water made its way to lower points.
Further evacuations ordered
This morning the Whakatāne District Council ordered residents of Gow Road in the town to immediately evacuate.
Civil Defence spokesperson Ross Boreham said the voluntary evacuation was a precautionary measure.
The floodwaters also cut road access for 1600 people in the Ruatoki area and 600 in Ruatahuna and Minginui.
Whakatāne district mayor Tony Bonne said two people had been flown out of Ruatahuna and there would be aerial food drops today. However the road to Taneatua was now accessible.
'Some places won't come back'
Surf life saving crews with inflatable rubber boats were drafted in to help rescue people.
Lifesaver John Hume said cars had been submerged, water was up to windowsills of houses and a mess of debris like timber, sheds, buckets and chairs floated through the streets.
Volunteer firefighter Steve Morrissey went door to door to make sure everyone was out.
"When this finally goes down there is going to be that much debris and silt and rubbish to clean up it is going to be horrible," he said.
"Some places won't come back."
It was estimated up to half the homes in the town were flooded.
Mr Bonne said Taneautua Lions Club had donated $10,000 for people in need, and a mayoral fund would be set up.
Most people had found somewhere to stay and only a few were in welfare centres. "Even strangers have taken people into their homes."
'They are practically all whānau'
It was not just Edgcumbe and its immediate surrounding areas that had been affected - about 2500 people remained cut off in the Whakatāne region.
Eighteen people, including children and a baby, spent a second night sleeping at a Whakatāne kura after being trapped by slips.
They were travelling to Murupara when mud and debris blocked them from continuing on or going back home.
Robert Eketone is the deputy principal of Te Kura Toitu o Te Whaiti-nui-a-Toi in Whakatāne.
He told Morning Report he had spoken to the people at the kura and everyone was fine.
He said a helicopter dropped off some supplies last night and locals had also provided some food to the group.
They were in good spirits, Mr Eketone said.
"They are practically all whānau so they're ... happy and they are dealing with the situation."
Farmers alongside the Piako River on the Hauraki Plains, about 200km north of Edgecumbe, were being warned to expect some flooding as water started going over stopbanks there.
The Waikato Regional Council said the overtopping of the stopbanks was part of the design of the flood system in the area.
It would go against the flood system to put sandbags on those stopbanks because it would increase the risk of serious flooding in the township of Ngatea, it said.
The council said the stopbanks were designed to contain floodwaters in ponding areas and prevent them getting onto farms. but the stopbanks could only contain so much.
'Stopbanks held up well' - Bay of Plenty Regional Council
Bay of Plenty Regional Council flood duty manager Graeme O'Rourke said ex-cyclone Debbie led to unprecedented river levels throughout the region. Some flows reached more than 30 percent above the one-in-100-year flood event most of its scheme's stopbanks were designed for.
Barbara Dempsey from Whakatāne's emergency operations centre told Morning Report the stopbanks held up well overnight.
Regional council staff used rocks to reinforce weak spots where water had started to seep through.
Water continued to pour through the huge break on College Street in Edgecumbe last night and repairs on that would not start on that until the weekend.
More water had been kept back behind the Matahina Dam.
Slips in Omokoroa
More slips have come down at Omokoroa, north of Tauranga, in the west of Bay of Plenty.
A 30m slip in Beach Grove yesterday caused stuctural damage to one house and the residents were evacuated as a precaution.
Overnight, a second slip in Beach Grove hit a small shed.
There were also slips in Kowhai Grove and McDonnell Street.
The Western Bay of Plenty District Council said there was no immediate threat to nearby houses.