But the commission was also being careful to ensure companies were able to make enough money "to invest in their networks and maintain quality of service, including reliability of supply".
The commission's decisions, which take effect on April 1, mark the conclusion of a two-year exercise aimed at preventing lines companies from exploiting their monopolies.
The commission employed Australian consultants Meyrick and Associates to assess the performance of all 28 lines companies plus national grid operator Transpower.
Its 101-page report ranks the companies in terms of efficiency and profitability.
Nine companies, including Counties Power and Taranaki's Powerco, were seen as making fat profits.
Seven, including Eastland Network, were assessed as having below average efficiency.
Nelson Electricity achieved the remarkable feat of having the fattest profits and being adjudged the second most efficient company.
By contrast, Buller Electricity was judged the least efficient lines company and had the third-lowest profit level.
The commission has now used that data to impose a form of differential price control on the companies.
From next year, lines companies will be allowed to increase their prices by the annual increase in the consumers price index minus a factor called X.
X has been assessed according to how much pressure the commission feels should be put on each company to improve its performance.
None was rated as having room for improvement in profit levels and efficiency.
But nine were considered to deserve a modest kick in the backside in one or other of those areas.
They were given an X of 2, meaning that if the annual rise in the consumers price index stays at 1.5 per cent, they will have to reduce prices by 0.5 per cent a year.
A further nine rated as being on a par with the industry average, including Horizon Energy in Whakatane, were given an X of 1, meaning they will be allowed to increase prices by 0.5 per cent annually.
Seven companies thought to be doing fairly well - including industry giant Vector and the Far North's Top Energy - got an X of zero, which means their prices will be able to rise at the same rate as the CPI.
Three superstar lines companies considered to be highly efficient and making low profits - Northpower in Whangarei, OtagoNet and Te Awamutu-based Waipa Networks - got an X of -1 which means their prices will be able to rise 1 per cent faster than the CPI.
The commission decided to hold off any final decisions on Transpower, pending a decision by the Electricity Commission on its investment programme.
It has been given an interim X of 1, allowing it to put prices up by 0.5 per cent.
This will apply for a year while further studies are carried out.
The commission will reassess companies' profitability and efficiency annually.
As well, they will continue to be rated on their quality of service, based on reliability and engagement with consumers.
Companies found to have breached either set of requirements will be further investigated by the commission and could be subjected to price, quality or revenue controls.
Rebstock said the variable X factor would give companies a continuing incentive to lift performance.
Better-performing businesses would be allowed to retain relatively more of the benefits of any efficiency gains they made. But business with below-average productivity or with relatively high prices would face tougher price restrictions.
Two out of line
The Commerce Commission has found that two electricity lines companies breached its interim rules on pricing.
But it decided that the breaches by Top Energy in Kaikohe and Nelson Electricity were the result of timing differences between movements in their costs and when they increased their charges.
As a result, it said, it would not impose price controls on their services.
The commission is also making further investigations into whether a further 15 companies have complied with the requirements.
Of the 29 lines companies, only 12 - including Vector, Counties Power, Eastland Network, Horizon Energy and Northpower - have been found to comply.
Read the Commerce Commission report
Herald Feature: Electricity
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