Power was due to be restored to all remaining customers last night, but Vector warned there could be some "isolated incidents" through the night as work continued.
The company also said hot water usage in parts of Auckland would be limited to prevent an overloaded network.
Vector spokeswoman Sandy Hodge said once Sunday morning's fire was extinguished, 65,000 customers had their electricity returned "relatively quickly".
"We had to check each cable and restore with safety in mind," she said. "This left 20,000 customers affected by the damaged cables."
Vector and Transpower met yesterday to begin a joint investigation into the fire's cause. Transpower chief executive Alison Andrew said the investigation was in its early stages. "We just don't know what caused it ... We're deeply regretful and sorry for the inconvenience and difficulty for people being without power ... it has a big impact on people.
"Our focus will be to work through the investigation to find out what happened and what we can learn."
Ms Hodge said a power outage in the Ellerslie area hours before the fire was not thought to be connected, but it would be investigated thoroughly before it was ruled out.
A number of homes lost power about 11.30pm on Saturday after a fault with an 11kV feeder at the Remuera substation. Ms Hodge said the feeder was isolated and switched to restore power to those affected. "Of course we'll look into this as part of any investigation," she said.
The investigation would also include an audit of assets at the substation by both Vector and Transpower, to identify any other areas that were vulnerable and where failure could result in similar outages.
Compensation for affected residents and business owners would be considered "once we have ascertained the cause", Ms Hodge said. Ms Andrew said it was "far too early" to comment on compensation.
She said Transpower had a comprehensive risk management system that included a risk register for assets like those within a substation.
Any lessons learned during the investigation would be included in asset management and maintenance plans.
Responding to claims that fire prevention measures were inadequate at the Penrose substation, Ms Andrew said the agency also had a strong fire management plan.
Auckland Fire Service assistant area manager Mike Shaw could not comment on the substation's fire prevention methods while the investigation was ongoing.
The Auckland Energy Consumer Trust, which owns 75 per cent of Vector, said late last night it had been briefed by the company.
Trust chairman William Cairns said: "The trustees are satisfied that all that could be done was done to restore power as safely and as quickly as possible, without fatality or injury."
Energy and Resources Minister Simon Bridges said the "significant event" had disrupted thousands of businesses and homeowners. "It's important there is a full inquiry to give the public confidence that risks to power supply are being adequately managed."
Insurance companies yesterday were gathering information and figures on power outage-related claims. Tower and AA had received a small number of claims so far.
Q & A
What happened?
Cables supplying power to parts of Auckland's eastern, central and southern suburbs caught fire at Transpower's Penrose substation on Sunday about 2am.
What areas were hit?
Remuera, Mt Wellington, Mission Bay, Greenlane, Orakei, Meadowbank, Kohimarama, Sylvia Park, St Heliers, Glendowie, Otahuhu, St Johns, Onehunga, Epsom, Mt Eden, Penrose, Te Papapa, Rockfield, Newmarket, Glen Innes and Ellerslie.
How many lost power?
At the height of the outage, 85,000 homes and businesses were without electricity.
What is happening now?
All affected customers had power back yesterday afternoon, but Vector said last night it was controlling hot water use in parts of Auckland to ensure the network did not become overloaded while two cables were still being worked on. It also warned last night that some customers might lose power at peak times.
We have engineers on board, firms say
Vector has hit back at claims that there are not enough engineers on its board.
Accountant Michael Stiassny, the chairman of Vector's board, said Wayne Brown's criticism was unwarranted.
In yesterday's Herald, Mr Brown, a former Vector and Transpower board chairman, criticised a lack of engineers on the two companies' boards.
He said this deficiency could limit understanding about the practicalities and dangers of the network.
Mr Stiassny, who is paid $198,450 for his role, said there were two highly experienced engineers on the eight-person board in Bob Thomson and James Carmichael.
Mr Thomson is an electrical engineer and former chief executive of Transpower. He was in charge during the 1990s when the company believed in more localised generation requiring less spending on major grid assets.
Mr Carmichael is a trained engineer with international experience including responsibility for energy assets and acquisition strategy for Power-Gen International Ltd and thermal and hydro power generation investment decisions for Ranhill Power Berhad.
"Their understanding - and robust questioning - of Vector's operations is beyond reproach," Mr Stiassny wrote in a letter to the Herald, published today.
Transpower has two engineers on its seven-person board - deputy chair Ian Fraser and Professor Jan Evans-Freeman, the Pro-Vice-Chancellor of Engineering at the University of Canterbury.
Chairman Mark Verbiest, a lawyer, said he disagreed with Mr Brown's comments.
"The board of Transpower has a wide skill mix including highly experienced practical engineering and electricity industry expertise."
-Nicholas Jones
The boards
• Vector: Michael Stiassny (chairman), Peter Bird, James Carmichael, Hugh Fletcher, Jonathan Mason, Dame Alison Paterson, Karen Sherry, Bob Thomson
• Transpower: Mark Verbiest (chairman), Ian Fraser, Jan Evans-Freeman, Abby Foote, Mike Pohio, Don Huse, Keith Tempest.
