According to Detective Inspector Roger Schranz, VicRoads crews were "rehabilitating" the stretch of the Western Freeway following an earlier crash in the area on Tuesday night.

"The bus has slowed down, as per the signs indicating the reduced speeds, then the truck has come around the corner and headed down towards the hill, it's seen the speed reduction signs and banked up traffic and collided with the bus, forcing the bus off the left hand side of the freeway down the cliff," he told reporters at the scene.

There are no details on the first collision, except that heavy haulage was required and it occurred "not far" from the site of the bus crash.

The VicRoads workers were "working on the side of the road in relation to rope barriers destroyed from the previous collision", he said.

Detective Inspector Schranz said it was "early days" in the investigation, noting the truck driver involved in the bus crash had not yet made any comments.

"The driver was conveyed to hospital in Melbourne," he said.

The school bus crash occured about 3.15am near the Condons Road exit - the bus, with 27 Loreto College Ballarat secondary students and four staff, was on its way towards Melbourne airport for a NASA space camp.

Two people, one a student, were flown to hospital in a serious but stable condition, while other passengers and the drivers were taken to the Royal Melbourne, Sunshine, Footscray, and Ballarat Base hospitals in stable conditions.

Grampians Health confirmed a "code brown" emergency is still active at the Ballarat Base Hospital, indicating increased pressure on emergency services, adding some students had already been discharged.

Loreto College issued a statement thanking emergency services and asking for privacy.

Diversions are in place while police investigate the crash, and drivers should avoid the freeway if possible.

More to come.

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