The rollout of the Mariyung Fleet - the official name for what used to be called the New Intercity Fleet - was meant to finish in 2021 with the South Coast line.

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The South Coast rollout was bringing 10-car trains during peak hour to eliminate commuter congestion - something promised an an election sweetener way back in February 2019.

But ongoing problems had seen the arrival date pushed back.

In May last year it was quietly announced that there was a new arrival date of late 2022.

But a NSW Parliament budget estimates session heard that COVID had created further delays with 27 trains still in South Korea and another seven yet to be constructed.

The parliamentary budget estimates session also heard that the timeline could have blown out by as much as a year, meaning the trains wouldn't arrive on the South Coast line until late 2023 at the earliest.

Also causing delays is the industrial stoush with the Rail, Tram and Bus Union over manning the new trains.

"A timeline for when the trains will start operating on the Blue Mountains and South Coast lines will be determined after more trains are delivered and further testing completed," a Transport for NSW spokeswoman said.

"A project of the scale of the Mariyung fleet has many moving parts, which means timelines can change to ensure the best and safest options are selected during design and construction".

The spokeswoman said that travel restrictions during 2020 delayed the delivery of specialist components and technology, as did the ordering of an extra 42 carriages in 2019 to service the South Coast line.

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