"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 firefighters had been doing further debriefs at the station. A formal debrief will be held tomorrow when the crew who attended were back on duty.
"I think the more you talk about it is the best for them," Mr Gardiner said. "A lot of guys are wanting to talk it through."
He said they would discuss any "learnings", going through the response step-by-step, and discussing whether they were happy with how they'd handled things and whether they'd do anything differently.
"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. They had to wait for rescue helicopters to arrive and take the patients to other hospitals.
St John territory manager John Stretton said, if a status one patient was taken to the emergency room, staff there would need to focus on that patient, so to send the other patients there would overload the emergency room.
Patient condition is rated from zero to five, with zero being dead.
Mr Gardiner said the firefighters could not cut patients out of the cars until the helicopter was there 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 fast-growing scrub fire.
A car hit a power pole on Fitzherbert Avenue in the early evening - the driver was unharmed, but the power pole was damaged. In the second crash callout, 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, which "had the potential to get large".
"This time of year's very, very dry," Mr Gardiner said.
Firefighters could see the fire growing as they drove toward it.
"We thought 'holy heck - this has got bigger in a short time and the flames are starting to take hold'."
An area was marked for a fire hydrant but did not actually have a fire hydrant. Mr Gardiner said it was likely the hydrant had been removed, but the signage hadn't, and he was following it up with council.
With nine "busy calls", extra staff had to be called in three times. Mr Gardiner dubbed the shift "a night from hell".
