Transport officials in the NSW government were warned a year ago that parts of the digital trains radio system that crashed last week were outdated, a confidential document reveals. 

The entire Sydney train network shut down last Wednesday, stranding tens of thousands of afternoon peak-hour commuters. 

It took more than an hour for trains to resume running after the digital radio system that signallers and train controllers use to stay in contact with crews failed.

The report by Transport for NSW, which is dated March 24 last year, found that components in the digital trains radio system were "obsolete".

"This has been identified as a priority, with funding reallocated to address this and staff training also being considered as a long-term mitigation measure," the report said.

Premier Dominic Perrottet this morning rejected suggestions the failure was predictable.

"Maintenance is not an issue … It was a unique situation that occurred," he told ABC Radio Sydney.  

"It has never occurred before.

"It's a digital system that went down. It's not an old system, it's a new system."

He said the fact NSW transport workers got the system back up and running in one hour and 15 minutes during peak-hour traffic was "positive".

A Sydney Trains spokesperson said it had identified in 2021 that parts of the system would become obsolete in the next five years.

"Transport for NSW and Sydney Trains are working together on a plan for the broader technology upgrade for train radio systems, while continuing to keep the current system operating reliably," they said. 

The spokesperson said the technology implemented in 2016 was very stable and used across railway systems in Australia and the world, and upgrades were regular and a normal part of ongoing maintenance. 

They also said that a back-up analogue radio was an approved alternative means of communication, and its use was not uncommon. 

It comes after reports that drivers will be given hand-held radios in case of a future communications failure.