Howell has not responded to interview requests.
He has posted CCTV footage of who he believes was responsible - and now the police have it too.
Police confirmed the "callous act" and say they are following strong lines of inquiry.
"The offenders' actions by preying on someone involved in a serious crash has only added to her trauma.''
Johnstone has no memory of the crash or what happened afterwards, Howell said.
She was cut out of the vehicle and yesterday was undergoing surgery in the hopes of fixing her badly damaged right arm.
Howell said while she was being freed from the car a "low life piece of scum" and his associates - including a woman - rifled through Johnstone's handbag, stealing her iPhone 6 and wallet.
The robbery was "totally beyond belief", Howell wrote in an emotional post on Facebook, which has been 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!"
As police arrived, the people fled in a silver Subaru Legacy, heading south down Cosgrave Rd and onto Clevedon Rd.
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 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.
"You sons of a bitch were spending her money before she even made it to the hospital," Howell wrote.
He managed to track Johnstone's iPhone, which had been thrown into a hedge of the medical centre opposite the Z station on Clevedon Rd.
It had about 30 missed calls from Howell on it, as he had been waiting for Johnstone to pick him up, Howell posted.
"It would have been constantly ringing in your hand, each time showing a photo of myself and my daughter. You heartless piece of crap!
"If I had lost that phone and lost Shevaughn that day you would have stolen more from us that you ever could ever comprehend - the sentimental value of the messages, photos to us are priceless and you took them from her when your human instinct if you had any should have been one of help."
Howell is desperate to find those responsible but said he felt blessed that he did not lose his partner that day, and that she could hold their 4-year-old daughter again.
"Wounds both physical and mental will heal in time, but now some of the little things that used to matter, just don't matter anymore...so stay safe and give your loved ones a hug as life for us feels just that little bit more fragile."
But his message for the thieves was not so gentle: "If you are reading this post let me assure you that you will be found, it is just a matter of time," he said.
"Watch your back as karma is a bitch and you are going to meet her head on!"
Police said they were called to a crash involving a car and a small truck at 1.15pm on Thursday, November 2.
The 39-year-old driver of the car was seriously injured and had to be removed from her vehicle.
"It has been subsequently established that whilst she was receiving medical attention a person or persons has removed her wallet and phone from her car,'' a police spokesman said.
"A credit card from the wallet was fraudulently used on eight occasions later that day."
Police are appealing to anyone who may have witnessed the incident to contact them immediately.
They are also looking at CCTV footage.
• Contact Papakura Police: (09) 295 0200 or CrimeStoppers Anonymous: 0800 555 111.
