NSW Treasury's chief economist says the last-minute decision to shut down trains last week could have cost the state up to $50 million - more than the $33 million his modelling suggested - because commuters did not have ample notice to make other arrangements.
Stephen Walters was asked by the Crown Solicitor's Office to model the cost of shutting down the train network to support a Fair Work Commission action against a union that did not plan to do so, a budget estimates hearing has been told.
Sydney commuters woke on February 21 to a city with no trains operating, on the same day Australia's borders opened and international tourists began arriving at Sydney Airport.
Mr Walters' modelling was just "contingency planning", Treasurer Matt Kean earlier told the hearing on Monday.
He says he found out about last Monday's 24-hour shutdown on the front page of a newspaper.
Premier Dominic Perrottet and Transport Minister David Elliott both said last week that was where they first learned of the train shut down too.
"Governments plan for all kinds of scenarios," Mr Kean said on Monday.
"It's entirely appropriate that we would do contingency planning for all scenarios, just like we did".
But he said he wasn't briefed on that specific contingency planning.
Treasury secretary Paul Grimes said he too "had not been aware of the scenarios that were being considered".
Mr Walters told the hearing that on February 15 he was asked by the CSO to model only one scenario; a two-week shutdown of the rail network, which would cost about $350 million.
He was not asked to and did not model the actions the union actually notified the commission it planned to take.
Mr Walters says the affidavit he was asked to compile about the economic impact of a train shutdown was a routine request.
He was not expected or required to inform ministers, or other employees within Treasury, about the request, aside from discussing it with his immediate team.
Asked why he conducted modelling on a two-week shutdown of the train network when it was not part of the union's notified action, Mr Walters referred the question to the CSO.
"We were asked to model a scenario, which we did," Mr Walters said.
The CSO "had indications from Transport" the train network could be shut but did not tell Mr Walters who would shut it down when it asked him to model the impact of doing so, he told the hearing.
He learned of the train shutdown in a similar fashion to the treasurer, premier, transport minister and most of the state's residents; from news reports on February 21.
Mr Kean told the hearing Mr Walters was only "providing general advice" in the affidavit, which his office did not receive until the day after Transport for NSW suspended services.
Opposition Leader Chris Minns says the circumstances surrounding the shutdown demonstrated "either massive incompetency, or they're not being completely truthful about who knew what when".
Trains resumed their full timetable on Monday, with almost 3000 services on the Sydney Trains network and almost 400 on NSW TrainLink regional services, Transport for NSW said.
"Sydney Trains and NSW TrainLink apologise for the inconvenience faced by commuters last week and we are committed to continue working with our employee groups to ensure we return to being a world class transport system," Sydney Trains chief executive Matt Longland said.
He said further meetings would be held this week and "contingency planning" was in place.
