A district judge urged by the prosecution to jail a man convicted of trying to pervert the course of justice ruled that a $6,000 fine was sufficient in the circumstances.

District Judge Kessler Soh agreed that while deterrence was the main objective in dealing with such cases, the sentence need not be a jail term but a fine "high enough to have a deterrent effect".

"Offences of perverting the course of justice are inherently serious in nature, which, if left unchecked, have the potential to undermine the administration of justice," said the district judge.

"Any attempt to influence the testimony of a witness, as in the present case, must therefore be met with a deterrent sentence," he added in judgment grounds released last week.

Ng Ang Heng, 59, an operations manager of a bus chartering firm, had admitted to trying to pervert the course of justice during the trial of his colleague Xu Yajie at the State Courts on April 28 last year.

He had sent messages through another colleague, Toh Lee Hong, to a witness, Ms Xu Xiaona, telling her to give a specific testimony.

At the sentence hearing last month, Deputy Public Prosecutor Jasmin Kaur called for a jail term of between six and 12 weeks.

She said Ng had shown "a great deal of audacity", sending the text messages as he sat outside the courtroom to influence the witness before she gave her testimony.

Citing precedents in which jail was the outcome, DPP Kaur said the offence showed total disregard for the administration of justice.

Ng's lawyer, Mr S. K. Kumar, argued, however, that the text messages were meant to affirm that he assumed liability in the subject matter of the case. He saidthe messages were not "harmful" and Xu was acquitted at the end of her trial.

The district judge held that deterrence had to be proportionate to the "culpability of the defendant and the harm caused by the offence".

He said the offences for which Xu had been tried were "not particularly serious in nature", and would ordinarily have been punished with a fine.

The judge found that Ng's degree of culpability for the offence was low, given he did not coerce, threaten or bribe the witness. "It was also not a case where he sought to influence the testimony of the witness for his own benefit," he added.

The judge noted Ng had previous convictions, but none similar to the present offence, and sentenced him to a fine of $6,000 - more than the $4,500 fine handed him in 2014 for making a false statement to the Manpower Ministry.

The prosecution is appealing.