The High Court cut the prison term of an abettor in a drug case from seven years to four years and six months, but had strong words for the district court, saying it had erred in jailing the prime offender for six years instead of 15 years.

Chief Justice Sundaresh Menon in judgment grounds released yesterday stressed it was the prosecution's duty to assist the sentencing court, which is exclusively tasked to decide the punishment.

"It is nevertheless incumbent on the sentencing court to evaluate the cogency of the position taken by the prosecution, which for its part, is obliged to place the relevant materials before the court to enable it 'to come to its own conclusion as to what the just sentence should be'," he said.

It was the first time a case has dwelt on the sentencing of a person who had abetted, from outside Singapore, the commission of a crime in Singapore since the provision was introduced into law in 2013.

K. Saravanan Kuppusamy, 35, had admitted to instructing, while in Malaysia on July 31, 2014, Kannan Reti Nadaraja to bring in 10.38g of heroin into Singapore by motorcycle. Kannan, 25, the principal offender, was nabbed the following day at the Tuas checkpoint.

He was convicted and jailed for six years and given five strokes of the cane on a reduced charge of importing 9.99g of heroin.

"Kannan was the beneficiary of a remarkably lenient sentence and this had been sought by the prosecution. I concluded in the circumstances that the sentence imposed on Kannan was wrong in principle," said CJ Menon.

A district judge ruled in April that Kuppusamy, as the abettor, had a higher position in the hierarchy and was more culpable than Kannan, a "mere tool". She was persuaded by the prosecution that it was a syndicated enterprise and ruled that Kuppusamy's sentence could not be less severe than Kannan's.

Kuppusamy, through lawyer Too Xing Ji, appealed this month, arguing that the judge had wrongly applied the principle of parity and had erred in finding the deal was part of a syndicate.

But prosecutors Wong Woon Kwong and Chan Yi Cheng countered that Kuppusamy was more culpable than Kannan, which explained Kannan's reduced charge.

CJ Menon said this "misses the point" and "rests on a misconception as to how the primary and secondary offenders should be punished in these cases".

He said Kuppusamy might well be more liable than Kannan but as both were charged with different offences, Kuppusamy's jail term had to be calibrated according to the punishment range prescribed for his offence and any attempt to connect the two should be done within such a context.

He said that as the offence Kuppusamy abetted was not close to the most serious under the Misuse of Drugs Act, he should then not be jailed at the highest end of the sentencing range for his offence.

Nonetheless, Kannan's sentence was too light and he should have been jailed for about 15 years, already well below the punishments for the most serious offences under the Act, which includes death and life imprisonment.

CJ Menon said the evidence was insufficient to show Kuppusamy was part of a syndicate.

He said Kuppusamy also pleaded guilty, saving costs of a full trial, which was a mitigating factor to cutting his jail term.

When contacted, an Attorney- General Chambers' spokesman said: "We are studying the judgment, and will take guidance from the High Court's approach towards sentencing offenders convicted under... the Misuse of Drugs Act."