NSW rail workers will refuse to staff the state’s new intercity fleet, citing safety concerns with the multibillion-dollar trains, escalating tensions between the Berejiklian government and rail union.

The Rail, Tram and Bus Union said there is a "design fault" with the $2 billion South Korean-built fleet, which has already had its rollout delayed across NSW. The government disputes the claim.

An artist's impression of the new trains that are being built in South Korea.

While the first two 10-carriage trains arrived in NSW in December last year for testing, nine months late, it is still unclear when the entire fleet is expected to be rolled out.

The union and government have been at loggerheads for more than a year over the new fleet and its potential impact on rail jobs, with the trains technically able to operate with a single driver and no guard.