Greenlea Rescue Helicopter, Hawke's Bay Helicopter Rescue Trust and the Palmerston North Rescue Helicopter all attended the incident.
Two more patients with minor injuries were flown to Bridge Pa aerodrome by private contractors but were not transported to hospital as hospital treatment was not needed.
The circumstances of the crash are still being established by three Transport Accident Investigation Commission investigators who arrived at the site this morning.
TAIC spokesman Simon Pleasants said the first thing the investigators will do is record the scene.
"They will map out the area, making sure they have an accurate record of the position of the wreckage. They are collecting all sorts of evidence which will likely take them all day and into tomorrow.
"The next work they will be doing is thinking about how to pick up the wreckage and take it down to Wellington," he said.
"We are probably going to pull it all together and lift it out by helicopter on to a truck, and then drive it down to our technical facility in Wellington.
"Once it is at the facility, we lay it all out again in the position we found it and work on it from there."
Pleasants said TAIC will also interview the station operator, the helicopter pilot, all people who were on board the helicopter and anyone who may have seen the helicopter flying.
"We will be seeking all extra evidence such as video footage people may have taken of the helicopter in flight.
"We also consider the weather, and we go into things like document records that have been kept by the operator on systems for safety and maintenance."
Pleasants said it hadn't been confirmed who the pilot was working for.
A Helicopters Hawke's Bay spokesman said the pilot has many years' experience flying helicopters.
"We have been in communication with the Civil Aviation Authority and Transport Accident Investigation Commission, both of whom will conduct a full investigation into the accident. We will be working closely with them and providing all the information required.
"At this stage it is inappropriate to comment or speculate on the cause of the accident."
He said their primary concern was the three injured patients and their families.
TAIC opens an inquiry when it believes the circumstances of an accident or incident have - or are likely to have - significant implications for transport safety, or when the inquiry may allow the commission to make findings or recommendations to improve transport safety.
