A six-month investigation into the fault that downed the Basslink subsea electricity and data cable connecting Tasmania to mainland Australia late last year has been unable to identify the cause of the cable cut.

The Basslink cable spent six months offline following the December 2015 fault, causing big problems for Tasmanian internet providers and end users.

Basslink has been working on a full fault diagnosis since the cable began operating again in June.

However, despite subsea cable experts Cable Consulting International spending more than six months undertaking forensic examination, lab analysis, and materials testing as part of its investigation into the issue, Basslink today said it still had no idea how the cut had occurred.

The official finding of CCI's report was "cause unknown".

Basslink said CCI had been able to determine that the fault had occurred within the cable - not at a joint, for example - and that there had been no pre-existing mechanical damage. It also said the operating conditions around the time of the cable failure had been no different to normal.

"The point of initiation within the fault path and the direction of propagation of the electrical failure could not be determined due to the severity of the fault damage," Basslink said.

Basslink chief executive Malcolm Eccles said it was "not uncommon" that the cause remained unknown, based on other similar outages.

“The silver lining in this whole exercise is that CCI found the other section of the cable it sampled to be in sound condition. This gives us confidence in the long-term outlook of the asset," he said in a statement.

Eccles said Basslink had handed over the report to Hydro Tasmania and the Tasmanian government.

He said the report supported Basslink's long-running argument that the fault was a "force majeure event".

"Given this, we would like Hydro Tasmania to recommence meeting its contractual

obligations to Basslink immediately," he said. Hydro Tasmania had stopped paying its fees to the cable operator following the downtime because it disagreed with the cause of the fault.

The report marks the end of the investigation into the fault for Basslink; it said it would not undertake any further testing.