Operations at Australia's longest continuously running mine site will be reduced to maintenance only from January 2015.

Two hundred jobs will go, adding to the 100 shed since two separate incidents killed three workers in December and January.

A rock fall in June was the final straw for owner Copper Mines of Tasmania, threatening to add three months to the cost of keeping 200 workers on half-pay. 

``The discovery of a rock fall in the mine's ventilation drive . . . was a bitter blow as we were preparing for a staged restart of operations following the long shutdown,'' CMT general manager Scot Clyde said.

``This has been a very difficult decision for CMT and for our parent company Vedanta, especially after the large financial commitment to support our workforce on standby pay.'' 

The news came two days after Unity Mining announced its Henty goldmine at Queenstown would close in 2015, with the loss of 150 jobs. 

Australian Workers Union spokesman Robert Flanagan said the news had devastated employees. 

``The workers have struggled for six months,'' he told ABC radio. 

``They believed that we were almost back at work and they were looking forward to it.'' 

In December, two maintenance workers fell to their deaths from a platform in a shaft. A mudslide killed a third man the following month.  At the time, 320 were employed at the mine. 

Fifteen jobs would remain for the maintenance of the site, CMT said, and exploration would continue. 

``It would be at least 18 months before any reopening of the mine could be possible if it is found to be technically and economically feasible to do so,'' Mr Clyde said. 

Tasmanian Resources Minister Paul Harriss said support services were being offered to affected workers and Premier Will Hodgman would announce more plans today.

Mr Harriss said the government had done all in its power to keep Mount Lyell open.  ``They've told me personally that we as a government could not have done any more,'' he said.

The news came as Avebury Nickel Mines announced it was looking to reopen an operation at nearby Zeehan, potentially creating 200 jobs.