It sank 5m beneath the surface with Aarush inside trapped by his seatbelt.
At his sentencing today Judge Blackie said he agreed with Macwan's lawyer that his "carelessness was minimal".
The police did not oppose the discharge without conviction and Judge Blackie said it was right for the police to bring the matter to court.
"It is not for the police to decide the outcome, but the court."
Macwan had previously told the Herald he would accept whatever punishment he was given because nothing would be as bad as a life without his son.
"Truly I'm not worried about what might happen. Whatever sentence might come - to me losing Aarush is such a big thing I can't feel anything - just give me any sentence. It doesn't make any difference because it's not going to be bigger than that."
He had relived the tragedy many times and wished he had done many things differently but knew that dwelling on what might have been would not help.
"Still, it's something as parents that haunts us."
The family had been in New Zealand less than a year before the tragedy but Macwan said there was no chance he and his wife would return home to India and leave their son behind.
If they had been allowed, the parents would have taken their son back to India for burial, but Macwan was forbidden to leave the country because he was facing the criminal charges.
"We just want to stay with Aarush.
"We don't want to leave New Zealand even when the case is finished because he's here. This is our home now and we don't want to leave him."