Recovering in her Auckland home, Johnstone said she was still grappling with how someone could do something so "callous".
The worst thing, she said, was that the person that robbed her had posed as one of her saviours, trying to get her out of the car before he came back to steal her bag.
"Normal people wouldn't do that, it's like it was some sort of game.
"You don't save a bleeding woman from a car crash and then rob her, you just don't."
She said she was glad someone had been arrested over the act and her fiance Murray Howell was also thrilled.
Howell had taken to Facebook following the incident to publicise CCTV security footage of a man thought to have been using the woman's credit card at a service station.
The robbery was "totally beyond belief", Howell wrote in an emotional post on Facebook, which was shared more than 10,000 times.
"We are on a very busy road at a major accident and even with her bleeding out and semiconscious this piece of trash thinks it's ok to rip her off!," he wrote.
Johnstone has no memory of the crash or what happened afterwards, Howell said.
The offender had rifled through Johnstone's handbag, stealing her iPhone 6 and wallet.
The wallet was emptied of cash and a Visa debit card, and thrown out the window, Howell said. The car then headed to the Z Kingsway gas station, where the thieves made a card purchase of $23.90.
Howell managed to track his fiancee's iPhone, which had been thrown into a hedge at the medical centre opposite the Z station on Clevedon Rd.
Howell believed the purchase was a test to see if the card worked, because they then headed out on a "shopping spree" at Z Papakura North, The Warehouse in Takanini and Grove Service Station, using PayWave until there was no money left.
Johnstone said she was getting her stitches out on Thursday. She was also undergoing physiotherapy to help her fully recover from her injuries.
Johnstone said the truck driver had been to visit her and she was expecting to catch up with him over a coffee in the near future.
"He was a really nice guy - he came and saw me at Middlemore Hospital after the whole thing happened."