Police were called to a Waitārere Beach address about 12.40am.
They found four people with gunshot wounds when they arrived.
Manawatū Area Commander Inspector Ross Grantham said a woman and two teenage boys had been taken to hospital in a critical condition.
The woman was taken to Palmerston North Hospital by ambulance and the two teenagers were flown to Wellington Hospital.
"Another young person who was at the address is physically unharmed and they are being given wrap-around support," Grantham said.
He said the gun was found at the scene and a forensic examination of the property would take place today.
Grantham said police were not looking for anyone else in relation to the shooting, and officers would be visible around the area while an investigation was under way.
"This is a shocking incident for Waitārere Beach and the district," Grantham said.
"This was a confronting scene, and I want to acknowledge the emergency personnel who responded to the call for help.
"We are still in the very early stages of our enquiries, but we are focused on understanding how and why this tragic event occurred."
He said police would release more information when it became available.
