Two migrant farm workers are dead and nine other people injured after a minibus crashed in to a tree.

The bus was reportedly attempting to pass another vehicle while travelling the Calder Highway at Carwarp, 30km south of Mildura in north west Victoria about 7.45am on Tuesday, when it lost control and hit a tree.

The passengers all worked on properties nearby, police say.

"We believe ... they're all international workers," Mildura highway patrol sergeant Mark McDonald said. 

Two people died at the scene and the male driver has been taken to hospital with critical injuries.

One passenger also sustained critical injuries and has been flown to a Melbourne hospital.

Three passengers with serious injuries were taken to Mildura Base Hospital and four more were taken in for observation.

Sgt McDonald said some of the workers were from Samoa and police would notify their next of kin via the consulate.

"It's difficult to see why it's happened," Sgt McDonald said.

"Obviously, the weather's fine, road conditions appear to be quite good, there's no indication there was any animals or anything like that involved."

The Calder Highway between Kulkyn Way and Brownport Road is expected to be closed for several hours so the major collision unit can investigate the crash.