Emergency services were called to the intersection of State Mine Gully Road and Old Bells Line of Road at Newnes at 3.50pm.

Three small buses were travelling in a convoy along the quiet dirt road, when the first bus slowed down at the intersection.

The second bus did not stop, crashing into the first one at a "moderate" speed.

Police activated a "major emergency response", with police and highway patrol officers from Lithgow, paramedics, firefighters, and the Volunteer Rescue Association sent to the scene.

Ambulances responded from as far away as Richmond and Bathurst.

Paramedics set up a triage area, however police said the worst injuries appeared to be cuts and bruising.

A NSW Ambulance spokesman said 66 people were assessed at the scene, with two children and one pregnant woman taken to Lithgow Hospital with minor injuries.

The remaining children, who were a wide range of ages from 5 to teenagers, were also taken to hospital in a local bus as a precaution, police said.

Vision from the scene showed a mini bus - owned by a company which provides school holiday care - was significantly damaged in the crash.

The windscreen of the bus had fallen out onto the road, while the front end was crumpled.

Police said 50 children and 18 adults had travelled to the mountains from St Marys in Sydney's west on a school holiday excursion.

Crash investigators will examine the scene to determine how the accident happened.

It is understood charges could be laid against the driver of the second bus.

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