"Tragically the three people were deceased when recovered from the pond by emergency services," police said in a statement.
"Police are working to support the family of those killed. WorkSafe has been notified."
A hearse leaves the Raetihi farm property this afternoon. Photo / Lewis Gardner
The Palmerston North Rescue Helicopter attended and a spokeswoman confirmed that just before 2.40pm a fourth person was being air lifted from the scene.
She said he was an "elderly man" and he was en route to Whanganui Hospital in a moderate condition.
She could not give more information about his condition.
The property where the car is understood to be submerged is less than a kilometre out of Raetihi's township.
Raetihi is about 87km inland from Whanganui and close to Tongariro National Park.
The property is 74.5 hectares and the owners operate a farm improvement business from it, offering diggers, trucks, bulldozers and fencing materials.
Tassia Allum, an employee at Caltex Raetihi, said around 11am she had heard sirens, with two ambulances, a fire truck and a police car all headed up State Highway 4.