The conversion of the Sydney Metro rail line, which has carried passengers since 1895, to driverless trains was a complex task and won't be completed on time, NSW Transport Minister John Graham said on Wednesday.
The work, which in September forced an estimated 60,000 commuters in southwest Sydney onto buses, was scheduled to take about a year.
Now the opening date has been pushed back until sometime in 2026.
Industrial action due to an ongoing dispute between the NSW government and rail unions over a new pay deal had been "very disruptive" to the conversion work, Mr Graham told reporters.
"We've seen significant impact - including from industrial action - 130 days that have been impacted in some way," he said.
The Fair Work Commission in February ordered industrial action be paused until July, amid hopes the dispute could be resolved.
The Electrical Trades Union, one of the unions involved in the dispute, has launched legal action in the Federal Court in a bid to overturn the decision.
Rail, Tram and Bus Union NSW secretary Toby Warnes said the government was blaming workers for its own incompetence.
"Once again ... using rail workers as the scapegoat for the blow-out in delivery time for yet another transport project," he said.
"The NSW government and Transport for NSW should stop playing the blame game every time there's a transport failure - they are the ones responsible and should take accountability if they can't deliver their projects on time or on budget," Mr Warnes said.
Stations on the southwest metro conversion have been fitted with screen doors and mechanical "gap fillers" providing a bridge between platforms and the driverless trains that will eventually pull up there.
Lifts have also been added at several stations to improve accessibility.
But testing will take some time, as more trains are run more quickly along the line to ensure safety.
Sydney Metro is the nation's biggest public transport project, comprising four lines, 46 stations and 113km of new rail.
The project also includes lines to the city's west and the under-construction Western Sydney Airport.
Services from Tallawong to Chatswood in Sydney's northwest which opened in 2019 were extended under the harbour and through the city centre to Sydenham in August.
That extension opened 15 days later than originally planned while awaiting approval from safety regulators.
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