Marlborough district council is warning some flood-damaged roads in the Sounds might not be repaired, where residents have boat access instead.
A council-led study will look at "all the options" from full repair and maintenance through to abandoning some roads where boat access is possible.
Severe weather events in the past two years have shut roads in the Sounds, in some cases for long periods and with no safe alternative route.
While many have been repaired, some routes are fragile, and residents were still being encouraged to use water taxis to reach their properties.
The Future Access Study will look at which of the worst-affected roads are prioritised in repair work, and the council is warning that some might get a lowered level of maintenance.
Marlborough mayor Nadine Taylor said after the severe weather of July 2021 road repairs costing $85 million had been well under way when flooding hit again in August this year.
"That's thrown us right back to worse than what we were in July last year."
Early cost estimates were $300-$400 million to fix the roads across Marlborough, she said.
The priority was to get some certainty around the cost, what the community needed in terms of access and how to get the roads resilient enough to cope with future bad weather.
"The study will scope out everything, from what it costs to put the road right back and maintain future resilience, through to 'are there areas that will be a lower level of service', right down to 'is boat access feasible'."
Boat was a normal form of transport across the Sounds, she said, with residents on D'Urville Island and Arapaoa Island having boat-only access.
"When we're faced with a three-hundred to four-hundred million dollar cost ahead of us we have to look at every option. But nothing's determined, we're just putting all the options into the study."
The government had been supportive on the cost of repairs in the past and the council would be seeking funding help this time round, she said.
"We know that the people of Marlborough can't afford the figure in front of us."
Flood damage had also closed the main highway between Nelson and Blenheim for six weeks, and Taylor welcomed the reopening on Sunday.
"We're thrilled, it's good news," she said.
More than $15m has been spent on fixing the road, with four significant underslip sites that needed retaining walls built, and 30 new culverts were installed on the severely damaged section.