The Rail, Tram and Bus Union's plan to deactivate Opal readers at NSW train stations next week is "potentially illegal" and dangerous, the state's transport minister says.
David Elliott says the union has been told to withdraw the notified, indefinite action, which is scheduled to begin on Wednesday.
If not, he has vowed to go after any members caught participating.
"It is potentially illegal and it is very dangerous," Mr Elliott told reporters after a meeting with transport department executives on Thursday.
"I will go to any court in the country to make sure that any person that is seen to be doing this is prosecuted, and that includes standing down staff.
"It is also ripping off the taxpayers of this state who paid for this equipment."
The union plans to leave station gates open as it did last month, but this time the Opal readers will also be deactivated, preventing commuters from tapping on, rather than giving them an option not to.
Mr Elliott said 90 per cent of commuters were still tapping on when the gates were left open.
Not all stations have gates, although the union action next week will also deactivate stand-alone payment poles at suburban stations.
The Opal system is operated by a private company and Mr Elliott said he planned to also seek advice whether the union action would result in the government having to pay any penalties under its contract.
Mr Elliott said the cost of the union action was hard to estimate given variations in the number of commuters from day to day, although he forecast it would cost "millions".
The RTBU is among unions that took NSW government rail entities, Sydney Trains and NSW TrainLink, to the Fair Work Commission (FWC) in a bid to keep negotiating a new enterprise agreement and modifications to a fleet of new intercity trains the union says is not yet safe to operate.
Premier Dominic Perrottet declared negotiations were over at the end of August, after a month of industrial action disrupting services across several days.
He threatened termination of an enterprise agreement if there was further industrial action.
Mr Elliott said the planned deactivation of Opal readers could hurt the unions at the FWC.
"This is proof the union movement has no interest in acting in good faith," he said.
RTBU NSW secretary Alex Claassens said the move was designed to turn up the heat on the government and senior bureaucrats.
"They are the ones responsible for this mess, they can now live with it," Mr Claassens said.
Sydney Trains said it did not consider the ban to be protected industrial action and would seek legal advice.
The unions and rail entities are set for another day of conciliation at the FWC on Friday, with another hearing unlikely to take place before next month.