NSW Transport Minister David Elliott says he believes the rail union's plans to switch off Opal card readers next week is illegal.

The Rail Tram and Bus Union (RTBU) announced on Wednesday it would shut down the Opal system across Sydney "indefinitely" from September 21.

The move is part of ongoing industrial action by the union over a dispute with the state government concerning train safety and pay rates.

Mr Elliott said the government had received legal advice suggesting the action was against the law and would take any worker seen switching off card reader machines to court.

"Prima facie we believe it's illegal," he said.

"I will go down to any court in the country to make sure that any person who is seen to be doing this is prosecuted."

When questioned about the legality of the Opal shut down on Wednesday, RTBU state secretary Alex Claassens said he was confident it constituted protected industrial action.

"As far as we're concerned, we're within our rights," he said.

"If they've got a problem with us, they can take us to wherever they need to take us to have that conversation."

The Opal system shut down is expected to make train travel free for Sydneysiders by default.

"If there is no way to tap on or off, you cannot be fined," Mr Claassens said.