The NSW government will take the rail union to the Federal Court to seek damages over their planned industrial action to turn off Opal card readers at train stations. 

On Monday, the Rail, Tram and Bus Union (RTBU) announced it would provide commuters with free travel "indefinitely" by turning off Opal machines from 3pm until 7pm each weekday from this Thursday. 

The action is the latest in a long-standing dispute between the government and the RTBU which has caused months of disruption to commuters. 

Transport Minister David Elliott has now announced the government will take legal action to seek damages and any loss of revenue over what he calls unprotected industrial action. 

"It is a fundamental right of a government and a fundamental right of taxpayers to expect their government to receive revenues when it comes to public transport," he said. 

"This action will make sure the taxpayers don't forfeit revenues they are entitled to receive." 

Mr Elliott said turning off the card readers could easily cost the government tens of millions of dollars. 

"This could be very expensive for the union," he said. 

"And suggestions from the union that they don't want to inconvenience commuters is BS." 

He argued that flicking the "emergency button" to turn off the gates at three quarters of NSW stations is unprotected industrial action, as it means staff are doing a task they don't do on a day-to-day basis. 

Sydney Trains chief executive Matthew Longland said only the 40 stations with Opal card gates, which are mostly in central Sydney, will have their readers turned off. 

He said this may mean commuters are stung by a default fare if they tap on at a suburban train station where readers are turned on and then can't tap off at the end of their journey. 

Commuters who usually receive a transfer discount by switching from rail to another form of public transport and those who use Opal gated car parks may also be impacted.

Mr Longland said Sydney Trains was working closely with Transport for NSW to try and automatically re-calculate fares when commuters can't tap off due to the industrial action. 

He also promised to work through fare readjustment requests as soon as possible, as some people could be hit by $16.80 default fares. 

"...We certainly won't be taking this out on customers." 

Mr Elliott urged commuters to do the right thing and still tap on whenever they can. 

Last time the union turned off some Opal card readers, 90 per cent of commuters still tapped on and off, the government says. 

The RTBU says the government's decision to take legal action will only waste taxpayer funds and they should instead negotiate fair conditions for rail workers. 

"The NSW government has shown time and again that they're more interested in using taxpayer dollars on legal fees than they are in reaching an agreement that guarantees the safety of commuters," secretary Alex Claassens said. 

The union has taken a variety of industrial action throughout the year over wages and conditions as well as safety concerns with the billion-dollar South Korean-made intercity fleet which is sitting idle. 

Mr Claassens says they will not allow the government to put unsafe trains on the tracks so have no choice but to take action. 

A spokesperson for the union said the upcoming action was protected as it was voted in favour by members and notice was given to the Fair Work Commission. 

Over the weekend Unions NSW declared it will campaign against the NSW government until the March 2023 election to address growing anger amongst workers across the state. 

Mr Elliot insists the rail union is trying to influence the outcome of the election by inconveniencing commuters and blaming the government.