A plan to outsource electrical jobs on the Greater Sydney rail network will lead to lengthy delays for commuters, a trade union warns.

The Electrical Trades Union NSW (ETU) also says a move to contract out RailCorp’s electrical maintenance instead of keeping jobs in-house has already been tried, tested and failed.

The union says 21 electrical workers from RailCorp have been told they will not have a job next year due a restructure.

The positions to be culled include emergency response roles, electrical manager and maintenance engineer.

ETU NSW Secretary Steve Butler said the job cuts – from Sydney, Newcastle and Wollongong – would lead to delayed responses to breakdowns for commuters.

“Past RailCorp reviews have confirmed that retaining electrical maintenance personnel within the company was the most efficient way to maintain assets and respond to critical incidents,” he said in a statement on Monday.

He said industry insiders had told the union that response times would be between six to 48 hours for major incidents that happened outside of business hours, including blackouts or people being trapped in lifts.

Mr Butler also claimed RailCorp tried to outsource electrical maintenance work on the Hunter line several years ago but ended up reinstating the positions in-house.

Comment has been sought from the office of NSW Transport Minister Gladys Berejiklian.