A balloon is being partially blamed for contributing to Sydney's transport chaos as the Transport Minister sought to explain what exactly unfolded on Friday night and Saturday that led to passengers being stuck on trains between stations and major delays across the network.

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In a statement to the Herald late on Saturday evening as delays continued, NSW Transport Minister Andrew Constance apologised for the chaos, saying "the unsatisfactory performance of the rail network over the past 24 hours is unacceptable. I apologise to our customers".

He said problems began when a metallic balloon got caught between wires on Friday night.

"It is my understanding [that on Friday] evening a metallic balloon found its way into a tunnel and caused an overhead electricity wire to short," Mr Constance said. "This led to a major incident in which Sydney Trains had to disembark passengers in a tunnel.

"This was coupled with a software failure at Transport for NSW that led to the rebooting of the entire IT network. There were elements of the system that failed to reboot properly, resulting in a major impact for trains customers throughout Saturday."

The "urgent" wiring repairs, conducted at Wynyard station, led to trains being suspended in both directions between Wynyard and Central earlier on Saturday morning.

Most train services were running on or close to time on Sunday morning, with limited exceptions for planned trackwork.

Seven News reported passengers had to evacuate their train at Wynyard and walk through the tunnel on Friday night.

Mr Constance said he was advised that the IT system was "regularly updated" and usually without a major problem.

"I have now requested the agency provide a list of measures to mitigate against a repeat of what we saw [on Saturday]," he said.

"I expect to see this advice quickly given the unacceptable inconvenience caused."

The minister said the state government was "transforming" the railway network from "one in the dark ages" to a "modern 21st century network".

It comes after the Herald reported in May that confidential documents revealed that Sydney Trains was suffering almost two serious IT incidents a day following a bungled $425 million technology overhaul by the state’s main transport agency, placing the rail network at greater risk.

This has led to the rail operator’s management being so concerned about Transport for NSW’s ability to deliver IT services that it commissioned consultants at a cost of $396,000 to carry out an independent review to find gaps and recommend fixes.

The minister told the Herald on Saturday that the upgrades had not been undertaken without some difficulties.

"This transformation is not easy, but given the growth in passengers' trips from 300 million per annum to 400 million within a 7 year period we are having to make major investments," he said.

He asked Sydneysiders for their patience and reminded them to not abuse front line staff when there were incidents on the network outside of their control.

"It’s important to respect them for the great work they do," Mr Constance said.

The minister has been away from the city and working in Bega this weekend, where bushfires have been raging in his own electorate. Despite this, the minister said he had been in regular contact with Secretary of Transport Rodd Staples, who he said was "coordinating the response to this matter".

Sydney's roads were heavily congested in the western suburbs on Saturday evening amid the chaos, which continued throughout the evening as multiple sporting events were underway in Sydney Olympic Park.

It led one Sydney Trains staffer to vent his frustration to passengers of a lack of information from management. "You won't see our management, but they want you to be reassured by me making announcements to tell you that we have no further information," the driver said. "Thank you."

The delays affected the entire network, with a travel alert informing passengers that "technical issues" were impacting the T1 to T8 lines and intercity services to Bathurst, Lithgow and Newcastle.

Travellers trying to avoid the train chaos by using ride-sharing services were also hit, with surge pricing being experienced in the vicinity of train stations in parts of the city on Saturday afternoon.

Passengers branded the situation a "massive system failure", a "meltdown" and an "utter shambles".