The 75-year-old Marinovich, who was returning from a bakery about 10.40am, immediately stopped.
Leong, who was crossing the road with the aid of her walking stick, was taken to hospital.
But she died later that day, leaving behind two of five surviving siblings.
Her family said she was a loved aunt and had recently become a great, great aunt.
Police, meanwhile, charged Marinovich with careless driving causing death, which carries a maximum penalty of three months' imprisonment.
He immediately accepted responsibility and at his first court appearance pleaded guilty.
In the statement to police made through his lawyer, Marinovich said he was rounding a corner when Leong "ran out onto the road".
Leong had crossed the centre line and continued walking into the lane when she was hit by the Porsche, court documents obtained by the Herald show.
She was struck by the left front corner of the car and thrown forward, landing on the road.
Marinovich offered to attend her funeral on August 25 but police told him to stay clear, while Leong's family also did not want to participate in restorative justice.
This month Marinovich was sentenced in the Auckland District Court.
"This is one of the most difficult sentencings that happens in this court and while it is an obvious thing to say, I am going to say it anyway: nothing that happens today can compensate the family, in any way, for their loss," Community Magistrate Jan Holmes said.
However, Holmes struggled to find a cause for the fatal crash.
"I am noting that there is no clear explanation as to how this accident happened," she said.
"What is clear is that there was no alcohol involved, there was no speeding, there was no medical event, you were not on the phone, visibility was good and there was nothing wrong with your car.
"So in terms of culpability I conclude that the accident was caused by momentary inattention with the most dreadful consequences."
Rachael Reed QC, Marinovich's lawyer, said her client was "genuinely remorseful" but argued his age meant a community work sentence would weigh more heavily on him than other offenders.
"He wishes he had seen Ms Leong or could explain more readily how he failed to see her. He is devastated at his error and recognises that the real loss is suffered by her family."
Marinovich also wrote a letter to Leong's family which was "filled with emotion", Holmes said.
"I have no doubt of your remorse and if you could explain the situation or change it in any way, then I am sure that you would," the community magistrate said.
"Clearly the victim was much loved and much missed. That is something that of course you will carry with you forever."
The Leong family asked the court for any fine imposed to be paid instead to the Breast Cancer Foundation.
"I think the money is much better spent with the charity rather than it going to the court," Holmes said of the $3600 donated in Leong's name.
Holmes also said a conviction was penalty in itself but disqualified Marinovich from holding a driver's licence for seven months.
Michael Reed QC, Rachael Reed's father, said in a letter to the court he had known Marinovich for 40 years.
"I have driven with him on numerous occasions. He is a safe, slow and careful driver and does not speed.
"He is a man of integrity, a hard worker and is very respected in the industry. I was present when he was awarded for over 40 years' service in the motor industry."
Jonathan Gooderham - a friend since 1972 - also said Marinovich was "a man of the highest integrity".
He said he had been on several test drives with Marinovich and always considered him "careful and above all, a safe driver".
During the 1970s Marinovich sold luxury European cars such as Daimler and Lancia. Since 1984 he has specialised in Porsche sales and has worked for the Giltrap Group for 47 years.
Giltrap's head of communications Shaun Summerfield confirmed the Porsche Marinovich was driving was a company-owned car.
"Mr Marinovich has not taken test drives or driven any company-owned vehicles following the ruling," Summerfield told the Herald.
Marinovich has only one other minor conviction, for failing to comply with a traffic sign in 1985.
