A mass power cut has left thousands of Waikato and Thames-Coromandel residents without electricity.

The cut stretched from Thames down to Waihi, Katikati and as far as Putaruru, while also affecting many farms and properties in East Waikato as well as eastern areas of Hamilton.

Transpower confirmed the power cut and Powerco's website says the fault will last until midday and has been caused by a "fault on national grid".

However, by 10am, most properties were back to normal with just Paeroa and Whangamata affected.

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Transpower said that the power cut was the result of a fault at a Hamilton substation.

A Transpower spokeswoman said they were beginning to restore power to some areas.

"We have an outage on the National Grid affecting the Waikato and wider surrounds.

"We are currently restoring power as we speak. Once our network is restored, it will take some time for the local distribution companies to restore theirs, given the size of the outage. We expect this to take 60-90 minutes to complete.

"Some areas will come on immediately, others will take a bit more time."

Civil Defence Waikato said there had been a major fault on the national grid this morning and warned power wouldn't be back on for hours.

Work was underway to get electricity back by lunchtime if possible it said.

It was a far wider issue than local supply companies, a Facebook post said.

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Transpower posted on its Facebook page that it was currently in the process of restoring power to regions affected by the unexpected cut.

"We expect power to be fully restored within the next 60-90 minutes," said the post, reiterating the spokeswoman's comments.

"We apologise for the inconvenience caused today."