Some rural roads in the Whanganui and Manawatu regions could be closed for more than a month, Civil Defence Minister Nikki Kaye says.
Ms Kaye is in Whanganui for an update on the city's state of emergency, after the area was hit by one of its biggest-ever floods at the weekend.
The flooding was a one-in-85 year event, and some people in isolated rural communities could be cut off for many more weeks while roads were repaired, she said.
"We're expecting, again this is very initial figures, that at least three quarters of those rural roads in the next month could have access.
"That's really important, that's really encouraging but, again, there are some parts of these roads that are really damaged and could take a longer period of time and we need to let those assessments happen."
South Taranaki District Council chief executive Graig Stevenson said contractors were making steady progress reopening roads but the estimated cost of repairs was growing.
"Well according to the engineers, who spent some time in the air yesterday doing bridge and road inspections, the $7 million we were talking on Monday is a minimum, so we've just increased that range to $7-$10 million.
"That's road and bridge remediation only, just in the South Taranaki district," he said.
Ms Kaye said she hoped about half of the people evacuated would be allowed to return home today.
Properties in Whanganui itself were still being assessed.
Whanganui mayor Annette Main said all properties with low-level damage had been checked, and about half of those residents should be allowed back in today.
But she it was a different story on the badly hit Anzac Parade, she said.
"I can't give you the numbers exactly on that but at this stage nobody has been allowed to go back into those homes in Anzac Parade to stay."
The council was organising a meeting tonight with Anzac Parade residents, many of whom still had not seen the damage to their homes.
Civil Defence controller Jonathan Barrett said the meeting, at Whanganui Girls' College, aimed to keep the residents up-to-date but also had a public safety message.
The Anzac Parade cordon, which was only one house deep, had been slipping and residents had been using side streets to access their homes.
However, the homes were still unsafe because of contamination and potential issues with electricity.
The cordon was likely to stay in place until at least Saturday, and residents should respect it until it was removed, Mr Barrett said.
Listen to Whanganui Civil Defence Controller Stuart Hilton
Ms Kaye visited the South Taranaki settlement this morning to get an official update on the clean-up but did not stop to talk with locals, such as grandmother Dot Bowlin, who was handcuffed by police during Saturday night's evacuation and said she was talking to lawyers about legal action.
Ms Bowlin's neighbour, 69-year-old Roma Brewer, alleged he was struck in the side of the head and said he wanted someone charged.
But Ms Kaye said evacuations were really tough and the law made it possible to forcibly evacuate people, if it was deemed someone could otherwise die.
Any Waitotara residents unhappy at their treatment during the evacuation of the village needed to take it up with the Independent Police Conduct Authority, she said.
Meanwhile, Red Cross secretary-general Tony Paine said the recent floods in both islands were the worst disaster to hit New Zealand since the Canterbury earthquakes, and it had launched an emergency appeal.
The scale of the damage was becoming apparent and a comprehensive recovery job would be needed, with thousands of people needing help to get their lives back on track, he said.
All funds raised would go towards helping people in Whanganui, Rangitikei, Taranaki, Hokitika and Dunedin.
Red Cross emergency response teams have already been helping Civil Defence nationwide.