During sentencing last month, Judge Neave referred to Hallwright as a contributor to society with a "spotless reputation" and "impeccable character" - comments which drew widespread criticism.
Mr Mander said the judge's remarks could not form the grounds for an appeal "in the absence of the ultimate sentence imposed being considered not just inadequate but manifestly so".
He said appeal courts were constrained in their ability to go beyond the presiding judge's factual findings - a "very significant obstacle" to a successful appeal.
"Please be assured that the question of appealing Judge Neave's sentence was the subject of anxious consideration by this office," Mr Mander said.
Mr Kim, who is recovering in Auckland Hospital after the latest operation on his legs, said yesterday he was very disappointed by the Solicitor-General's decision.
"I lost two years of my life. Two years. I need six or seven more operations.
"It's just hell."
He would now consider civil action against Hallwright for damages.
"But the money is not important. If he came to me and said sorry, but he never ever came to me and he never ever apologised to me. That's the main thing."
Auckland Council for Civil Liberties president Barry Wilson, who wrote a letter to the Solicitor-General in support of the appeal, said the decision not to proceed was a "cautious approach".
"It's certainly no comfort to Mr Kim, who's now in hospital having yet another operation as a result of the injuries inflicted on him when he went under Mr Hallwright's car. It is not a satisfactory result and it certainly won't overcome public disquiet and dissatisfaction at the sentence."
He said Mr Kim could file a civil proceeding against Hallwright but it would be "quite a difficult hurdle".
Hallwright's lawyer, Paul Davison QC, said the Solicitor-General's decision was "totally predictable" and he didn't agree that the sentence was "undoubtedly lenient".
"[The case] has been really badly represented in the media - it has been presented as a hit-and-run case, but in fact Mr Hallwright was endeavouring to get away from what he thought was an attacker, namely Mr Kim," he said.
"[Mr Kim's] injuries do not indicate any intentional act on Mr Hallwright's part and, as a result, [Judge Neave] regarded it as a low-level offence and took into account Mr Hallwright's excellent record."
