Wairoa is completely cut off overnight and only has one day worth of food and enough drinking water for two days, Hawke's Bay Civil Defence says.
In a statement, the Civil Defence branch said the town had lost lifelines to Napier and Gisborne, including power, phones, internet and roads.
A National Emergency Management Agency (NEMA) representative was on route to Wairoa via helicopter from Napier overnight to support the team and the response effort.
With power restored to most of Wairoa by 5pm, with the exception of Mahia/Tuai, the key concern for the welfare of the community was be dwindling food and water supplies, Civil Defence said.
Controller Liz Lambert said that with the loss of roads, they would be relying on supplies coming in by air.
"Wairoa only has one day's worth of food, and enough drinking water for two days. We have made a request to NEMA for enough food and water to supply the district for seven days."
Much of Hawke's Bay remained flooded as the region braced for continued rain, Civil Defence said.
MetService issued an Orange Heavy Rain Warning for the 21hr period from 1pm Tuesday 14 February to 2am Wednesday, 15 February.
Evacuations in the wider Hawke's Bay on Tuesday took place in Ruataniwha, Waihirere and Ormond Rd, Haumoana, Eskdale, Taradale, Porangahau village, Waipawa township, Waipukura, Awatoto, Te Awa, Brookfield and Wairoa.
Police and FENZ have carried out numerous rescues and continue to respond to stranded residents, according to Civil Defence.
Evacuation Centres were activated at Taihoa Marae, War Memorial Hall and Presbyterian Hall. An Evacuation Centre in Nuhaka has been established at the Mormon Church.
Evacuation centres are in operation in Central Hawke's Bay, Hastings, Napier and Wairoa with additional sites being added as required.
In Hastings and Napier, the cause of power outages has been linked to the flooding of the Redclyffe substation causing the Transpower network to go down, Civil Defence said.
"Unison reported outages for 60,000 customers across Hastings, Havelock North, Napier, north along east coast to Tūtira and south to Waimārama. It is expected to take some time before power is fully restored across the region.
A number of the region's cell towers are being operated on battery supply allowing some network coverage although this is still intermittent. Mobile communications are still out in Wairoa with response teams relying on radio and FENZ communicating via satellite.
A number of bridges remain impassable and there is still no access between Hastings and Napier.