Whanganui senior fire station officer Craig Gardiner said the incident had been traumatic for everyone involved, including firefighters who were related to people involved in the crash.
"We never like to go to a double fatality -- even more so when it's relations," Mr Gardiner said.
There were two new members of the rescue truck, and it was their first time cutting someone out of a car, as well as the first fatalities they had dealt with.
"It is something they'll never forget ... something they'll carry through their whole career," he said.
A debrief was held on-site and there were further debriefs at the station. A formal debrief will be held today when the crew who attended were back on duty.
"The more you talk about it is the best for them," Mr Gardiner said.
"A lot of guys are wanting to talk it through.
"You've got to spread your resources between both of those vehicles, and you've got to make life-and-death decisions ...
"We made the right ones.
"It's always a worry, to make sure that you give them the best opportunity to live. We take that very seriously -- we do do the best for people in those situations."
Events on Saturday were made more difficult because they could not send all the patients to Whanganui Hospital, and they had to wait for rescue helicopters to arrive to take people to other hospitals.
St John territory manager John Stretton said if a status one patient was taken to the emergency room, the staff there would need to focus on that patient, so to send the other patients there would overload the emergency room.
Mr Gardiner said the firefighters could not cut patients out of the cars until the helicopter had arrived and was ready to transport them due to the nature of their injuries.
His team was at the scene for three hours.
Support procedures had been put in place for those who attended, he said.
Firefighters also attended two minor crashes on Friday night, as well as a scrub fire.
A car hit a power pole on Fitzherbert Ave in the early evening -- the driver was unharmed. In the second crash call-out, about 11pm, a car had hit a bridge and the occupants had run off. Two fire trucks were tied up with a scrub fire in Castlecliff.
