By Damian McIntyre, Australian Broadcasting Corporation

Repair crews are still unsure where the fault in Tasmania's vital undersea cable is days after cutting it, Basslink chief executive Malcolm Eccles says.

A fault in the cable has plunged Tasmania into an energy crisis compounded by record low rainfall which left the state's hydro dams critically low.

The Basslink cable was bringing about 40 per cent of Tasmania's power to the state but a fault put it out of action in December.

About 9:15pm on Friday, Basslink severed the cable as it began repairs. It is not expected to be fully fixed until May.

Mr Eccles told 936 ABC Hobart the fault was somewhere between the cut 89 kilometres off the Tasmanian coast and Victoria.

“We don't know until we actually start to do the next phase of testing,” he said.

“We need the weather window to be able to lift the cable to put it on the deck of the vessel and then we have to carry out a number of tests.

And only then after we've done those tests will we know if we're 200 metres, 500 metres, a kilometre [from where the cable was cut].”

He said the vessel being used in the repair job was very expensive.

“If you've got a submarine cable which is 300 kilometres and you need to fix it it's $100,000 a day for a vessel, minimum,” he said.

“The repair costs are phenomenal.”

Basslink also carries fibre optic cable as well as its power infrastructure.

It is used by a number of non-Telstra affiliated internet service providers (ISPs), but that service stopped when the cable was cut.

That left about one third of Tasmania's internet users with very slow or “unusable” internet speeds.

Mr Eccles said his company's primary business was power and it may reconsider its telecommunications role in the future.

“We've always said it's got to be a viable business for us, the moment it becomes unviable…then it would be unfair to our shareholders to actually continue.”

That could also mean a price increase for customers.

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