The Belair line will also close over the holiday season, but only from midnight on Christmas Day until the first service on December 30.
The full length of the Seaford line will be closed on December 27 and 28 after 8pm, and then again for the full days of January 2 and 3.
In addition, there will be a partial closure of the Seaford line between Brighton and Seaford from January 4 to 25.
The State Government is promising substitute buses on both lines during the closures.
Maintenance on the Belair line includes bank stabilisation, signal and communication cable testing and civil works at three level crossings.
But the problems on the Seaford line are more extensive, with the Government deciding to replace faulty electrical wiring which has plagued the line over the past 18 months.
The Government recognised the problem earlier this year, and signalled then that “kilometres” of cable would need to be replaced.
Today, Transport Minister Stephen Mullighan said the Government originally believed it could manage maintenance with night-time closures, but that wasn’t possible given the extent of the problem. About 35 kilometres of the cable would need to be replaced.
He said it was “incredibly disappointing” to close the line, even though it was the quietest time of the year.
The Government was “absolutely furious” to have spent hundreds of millions to provide a good service to commuters only to be “let down” by a supplier.
“We are aiming to have the services back in time for Australia Day and the new school year,” he said.
He said the Government had decided to replace faulty overhead wiring which had failed twice on the Seaford line, but the bill would be paid by the contractor, not taxpayers.
“When the catenary wire split at Seaford in June 2014 it was thought to be an isolated incident,” he said.
“After a second split at Christies Beach in May this year, more extensive investigations of the wire and manufacturing process have been undertaken, and instead of replacing the sections first thought to be affected, the decision has been made to replace the entire lengths provided by the manufacturer.
“This will take much longer than previously estimated and to minimise risks to ongoing services, the closure in January will reduce the chances of ongoing interruptions to commuters.”
The Government ensured third-party oversight of the manufacturing of the new cable to make sure it was fit for the job.
Mullighan said the original contractor, Laing O’Rourke, would take responsibility for the cost of replacing the wiring as well as communication with passengers.
Laing O’Rourke said its supplier, Olex Australia, had provided the company with a faulty batch of catenary wire “which we are dealing with through other processes”.
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