A St Johns spokesmansaid the ambulance was called by a member of public just after midnight and all four passengers were immediately transported to Hawke's Bay Hospital with two in a serious condition and two with minor injuries.
It is understood they were thrown from the vehicle as it rolled down the bank into the dam.
Mr Oliver said based on the skidmarks it looked like the car lost control and went along the grass beside the road for a while before going through the fence and into the dam.
The distance from the road to the dam was described as being about two lengths of power poles.
Hastings Fire Station fire officers assisted the Otane team with a rescue truck and appliance.
Mr Oliver said the vehicle was unable to be salvaged at the time of the accident due to its location in the dam and the darkness.
"From the road it looked like it landed in the grass as there was a lot of duck weed around but that wasn't the case."
A police spokeswoman said inquiries about the crash were still ongoing.