Claims that the first doctors' strike in NSW since the 1990s, beginning Tuesday, would not impact patient care were dismissed by the state's health minister on Monday.

"You simply can't have thousands of doctors threatening to walk off the job out of hospitals without having an impact," Ryan Park told reporters.

"We've already started cancelling surgery ... our emergency departments are going to be running on reduced staff."

Premier Chris Minns said doctors could face consequences for defying the industrial umpire's orders not to strike.

"It's dangerous for patients, and it's risky for doctors," Mr Minns said.

New graduates up to specialists are expected to walk off the job for three days as part of a campaign to have NSW match interstate wages and conditions.

First-year doctors earn about $38 an hour but can collect $45 per hour in Queensland before considering penalty rates and extra leave.

The union says action is necessary to address "critical doctor shortages" and ensure everyone. "They have failed to do that," he said.

With AAP.