Powerco estimates it will cost at least $2 million to repair the damage caused by last week's storm.
Operations manager Phil Marsh said the Central Plateau remained the worst affected area where heavy snowfall and high winds brought down poles and lines.
"People are saying it's the worst snowstorm in that region since 2002. On that occasion our network received massive damage - and this is just as bad."
Power was restored to the township of Mangaweka on Sunday although some rural and remote areas remained without power.
Mr Marsh said: "The complexity of the damage to more than 50 poles and kilometres of downed lines in the area has meant that work is slower than we anticipated.
"About 800 customers in the Central Plateau remained without power and it could take a few more days to restore supply to some very rural customers.
"The team working on the storm damage have been working extremely hard in difficult conditions. We have more than 220 additional crew working in the field to restore power across the Western area," he said.
Some customers in the Taihape region remained without power on Sunday and they could have to wait a few more days.
"We have identified those customers we anticipate will be worst affected. While some may have their own generators, our staff are contacting everyone to see if they need assistance," Mr Marsh said.
"We are gradually mopping up problem areas in Whanganui, Wairarapa, Manawatu and Taranaki.
"Good progress is being made restoring high-voltage supply. However, some people might still have low-voltage problems."
Mr Marsh urged people to remain wary of downed lines. Anyone coming across them should keep well clear and call emergency services on 111 or Powerco on 0800 27 27 27.
"Use these numbers for emergency calls only. People have been calling our 0800 for information on outages which has been clogging our emergency lines - they need to call their retailer or check our faults page at www.powerco.co.nz."