Auckland suburbs that have been without power for more than a day and a half face a further delay until the lights come back on.
Late this afternoon, Vector said the timing to repair and restore cables had been pushed out by two hours due to operational requirements by Transpower.
Crews now hoped to restore power to customers in St Johns at 10pm, Sylvia Park at 1am, Carbine Road at 3am, Mt Wellington at 4am and Remuera about 7am.
Watch: Power still out in Auckland
Meanwhile, strain on Auckland's power grid has caused further outages in areas where electricity had previously been restored.
Vector said 1500 customers who had their power restored after yesterday's blackout had since lost power due to pressure on the system.
The areas where power has dropped out again included pockets of Glendowie, Grafton, Eden Terrace, Newmarket and Newton.
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Vector spokeswoman Sandy Hodge said the situation was "evolving" as power was back-fed to areas from elsewhere in the network.
"Where we've managed to back feed, in some situations with load coming up, the load's become a bit much and we've had to drop off some of those people on that feeder," Ms Hodge said.
"It's desperately trying to balance the load we've got, because we've back-fed it all over the network while we get these damaged feeders replaced."
About 18,000 customers in eastern Auckland were without power this morning, down from 85,000 at the peak of the blackout yesterday.
Areas still without power this afternoon include Orakei, Sylvia Park, Mt Wellington, Ellerslie, St Heliers, St Johns and parts of Glen Innes and Remuera.
There had been concerns that heavy winds, forecast to gust up to 100km/h this afternoon, would add to the strain on the system.
However, Ms Hodge said the weather had not affected work to restore power and replace damaged cables when she visited the Penrose substation about midday.
"Mercifully, there was not driving rain -- a bit of wind, but it wasn't enough to stop work, and that's what we're hoping for."
The worst of the weather was expected to have passed by 1pm, so it was hoped there would be no further outages.
Ms Hodge said there had been some outages in Green Bay, Riverhead and Titirangi. However, the outages were due to high winds and were unrelated to yesterday's fire.
The cause of yesterday's fire at the Penrose substation had not yet been established and was still under investigation, Ms Hodge said.
Asked whether customers would be able to claim for compensation from Vector, she said: "At this stage we need to find out what the cause is first."
The priority was to get the power back on, Ms Hodge said.
Vector earlier said work on the fire-damaged cables at the Penrose substation was progressing well.
"All available crews are working on these cables in order to repair and then reinstate them," Ms Hodge said.
"In the meantime, customers that have been restored in the affected areas are asked to conserve power as much as possible so we can back feed more areas."
AUCKLAND'S BLACKOUT SPOTS
Areas still without power:
* St Johns
* Sylvia Park
* Mt Wellington
* St Heliers
* Parts of Glen Innes
* Parts of Remuera
Areas where power has dropped out again:
* Parts of Glendowie
* Parts of Newton/Eden Tce
* Parts of Newmarket/Grafton
Meanwhile, all hospitals affected by yesterday's powercut are expected to be fully operational by tomorrow.
Private hospitals Mercy Hospital in Epsom and Ascot Hospital in Remuera were both affected by the power outage yesterday and were running on back-up power.
Chief executive Dr Geoff Sparkes today said Mercy had power restored around 5.40pm yesterday and surgeries commenced on schedule today.
Full power had also resumed at Ascot, however on advice from Vector, around 40 surgeries scheduled to take place at the hospital today had all been rescheduled, he said.
"Vector said that the power could go off at any time so we shouldn't rely on it.
"We had to make the decision late yesterday afternoon about surgery today in order to make sure that patients weren't turning up on the door step only to be turned away."
Vector advised them to expect full, reliable power at the hospital tomorrow, Dr Sparkes said. The delayed surgeries had not had a domino effect on other scheduled services, he said.
An Auckland District Health Board spokesman said Auckland City Hospital had not been affected by the outage and Greenlane Clinical Centre was operating as normal with backup power sources. Surgical lists were not affected.
The hospital's emergency department experienced usual numbers of in-patients between the 24 hours from 7am on Sunday until today, he said.
Family and friends of those in the community whose health may be vulnerable and were affected by the power cuts were advised to check on them and if they had concerns to call Auckland City Hospital's clinical nurse advisor on 021 938 695.
Auckland Surgical Centre manager Michael Quirke said they delayed "a couple" of surgeries this morning at their Remuera centre due to the threat of potential power cuts during peak hour this morning. "We essentially just played it safe. We've been online since yesterday."
The surgery reverted to back-up power when the cut struck yesterday, however there were no patients on-site at the time, he said.
Southern Cross spokeswoman Amy Bourke said generators at their Brightside Hospital in Epsom kicked in when the power cut struck and full power was restored at 5.30pm yesterday.
No surgeries were delayed as a result, she said. "We've had no impact whatsoever."
Gillies Hospital manager Trisha Dunn said they lost power yesterday but the cut did not affect them as they were closed in the weekend.
Patients were warned yesterday that if power did not resume today then their surgeries would be postponed. However that was not the case, she said.
5 Top power outage tips
1 For light, a torch is safer than candles.
2 Switch off sensitive electronics such as the TV, computer, stereo and video recorder until after the power is restored. These can be affected by a power surge when power comes back on.
3 Keep the fridge closed. Food in refrigerators and freezers will last longer if the doors are closed.
4 Turn off all appliances, including the stove, kettle, and heaters. This ensures they don't come back on when no one is home.
5 Street lights and traffic lights may not be working.