The Rail, Tram and Bus Union failed to reach a breakthrough in emergency talks with the NSW government and will not provide the extra services needed for a marquee weekend of sport.
In Sydney there are NRL and AFL finals - including the Swans' sold-out game against Port Adelaide at the SCG - and the Bledisloe Cup rugby union fixture.
Business Sydney said conservative forecasts showed the events were worth $21 million to the city's economy.
"Such major public transport inconvenience could not come at a more critical time," executive director Paul Nicolaou said.
"This boost for struggling businesses will be severely dampened if footy fans are impacted by this industrial action."
Business Sydney says the rail disruptions could not come at a worse time. (Bianca De Marchi/AAP PHOTOS)
The union is also threatening to shut down the entire T3 train line - from Liverpool in Sydney's southwest to the city centre - if the government pushes ahead with plans to convert part of it to driverless metro standards.
The conversion requires closing the Sydenham-Bankstown section from September 30, but Ms Haylen warned union work bans could delay the construction altogether at a cost to taxpayers of $100 million a month.
The rail union said the conversion was unsafe and represented a different beast to newly constructed metro lines elsewhere in Sydney.
Explaining the government's decision to make travel to and from the football free, Transport Minister Jo Haylen invited the RTBU to keep negotiating and find an outcome that worked for NSW.
"We know that this weekend is going to be difficult for passengers as a result of industrial action … we're going to open the gates and deliver free travel for passengers this weekend so tens of thousands of families across our city can get to the footy," she said.
"We're hoping that the RTBU will come back to the table and lift bans to get Sydney moving this finals weekend and build the infrastructure we need."
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