A multi-vehicle crash has left a child dead and multiple people injured in Sydney's north-west.

Police said a Holden Commodore and several other vehicles collided on Windsor Road, in Baulkham Hills about 7.15 on Saturday night.

An adult and a child are in a critical condition and have been transported to Westmead Hospital.

A search is under way to locate the driver of the Holden who allegedly fled following the crash.

Windsor Road is closed in both directions at Baulkham Hills near the M2.  Motorists are advised to avoid the area.

The drivers involved will undergo mandatory testing.

Bus routes M60 & M61 are diverting away from the area, missing 10 stops each way.