The Australian Energy Regulator, an arm of the Australian Competition and Consumer Commission, has been told to conduct a further review of the planned spending by electricity companies. Since the power companies are monopolies, their prices and spending plans are subject to scrutiny and approval by the government.

In Friday's ruling, the tribunal said the regulator must review a range of assumptions and spending allowances that were included in its decision last year that saw prices fall. The tribunal said it found in favour of the power companies on some issues and the regulator on others, but given the complexity of the ruling, the final effect on prices will now take several months to finalise.

Ms Conboy said a detailed review of Friday's decision will now be undertaken, and a further legal challenge could not been ruled out, with legal advice likely to be sought.

The power utilities argued the size of the cuts imposed on their spending plans were too steep and would compromise network reliability, raising the prospect of power interruptions and failures. As a result, they took legal action to force the regulator's hand.

A challenge was also launched by the Public Interest Advocacy Centre, which argued the AER's original pricing decision was too generous, and that power company outlays should be reduced further, which could have cut household power bills by up to another $300 a year. This was opposed by the networks, which argued for further spending approvals which would have added an estimated $500 to the annual power bill.