NSW Police have said they averted a potential disaster after a school bus driver was caught more than four times the legal alcohol limit.

The 55-year-old man recorded a blood-alcohol reading of 0.09 when he was breath tested by police at Dunedoo in the state's central west last Friday.

Chief Inspector Jeff Boon from Mudgee police said the man's ability to drive safely would have been seriously impaired at that level.

The limit for professional drivers, including bus drivers, is 0.02, he added.

"Even at 0.05 your chances of having a crash are doubled so exponentially the danger increases as you go up, and what you've got to bear in mind is that for a public passenger vehicle, for a school bus [driver the alcohol limit] is 0.02, and it's 0.02 for a reason," he said.

Chief Inspector Boon said police would also allege the driver was on his way to pick up a group of schoolchildren when he was caught.

"We've literally only averted a potential disaster here by a matter of moments," he said.

"The reason we have the 0.02 limit for professional drivers is so that they do not drink at all before they get behind the wheel.

"It's not acceptable for people that drive big trucks or school buses or coaches to have any level of impairment. You have other people's lives in your hands."

The man has been charged with mid-range drink driving and has had his licence suspended.

He will face Dunedoo Local Court later this week.