Powerco says sections of its network in South Taranaki and Whanganui will need to be rebuilt after a severe storm across central New Zealand on Saturday.
Powerco says damage to the network is severe, with 600 overhead lines down and a number of power poles damaged.
General manager Andrew McLeod says it's likely about 4500 properties will remain without power on Sunday night.
The areas most affected include Patea, Waitotara, Waverley, and remote areas between Whanganui and Raetihi.
Mr McLeod says it could take another three to four days to restore electricity to areas with extensive damage, or where access is difficult.
He says 170 staff were working to restore power on Sunday and field staff will be working overnight.
He says 200 staff will be working on Monday as crews arrive from other regions.
The number of properties without power has dropped from 30,000 on Saturday morning to around 4500 on Sunday afternoon.
Mr McLeod says all downed power lines should be treated as live.
He says the immediate focus is restoring services and Powerco will count the cost later, but he says it will be significant.
South Taranaki District Council has set up a drop-in centre at the Patea Old Folks Hall for people needing help or advice after the storm.
Farmers brace for water shortages
Farmers in some parts of parts of Taranaki and Whanganui may face days without power.
Federated Farmers spokesperson Derek Gibson says the top priority for its members is animal welfare, with many stock at risk of water shortages.
He says some farmers will be forced to dump milk which can't be collected.
Mr Gibson says Hawera Volunteer Fire Brigade has offered to transport stock water for farmers running dry.
Whanganui District Council has imposed water restrictions on the city after power was cut to its pumping stations.
It says power has been restored to two bores, but it does not know how long it will take for services to be fully restored.
Although the water is likely to be discoloured and at low pressure, Mayor Annette Main says it is safe to drink.
In the meantime, the council says people should not wash their cars or use sprinklers in their gardens.
Civil Defence Controller Kevin Ross says the fire service in Whanganui dealt with 66 major property damage or 'life at risk' events between 6.00am and 2.00pm on Saturday.