UPDATED: 3.24PM The Hawke's Bay Expressway has re-opened after a young boy was killed in a crash earlier today.

The boy was in the back seat of a Mazda car, when it rear-ended another car that had stopped in a line of traffic at roadworks on the Pandora Bridge.

His mother and younger sister have been taken to Hawke's Bay Hospital, where they are both in a serious condition.

Three occupants of the other car, including two children, have also been taken to hospital with various injuries.

The Serious Crash Unit's investigating.