A police sergeant who illegally accessed the Ministry of Home Affairs' computer system to find out if his wife's former lover had filed a report against him was jailed for 18 weeks on Monday.

Lim Ming Kian, 30, of the Central Police Division's Compliance Management Unit, pleaded guilty to 12 counts of knowingly misusing a computer system belonging to the police. He secured unauthorised access to data from the Frontline Officers' Computerised System (Focus) between July and November 2011. Focus enables authorised officers to lodge police reports and carry out searches on reports filed by members of the public.

Deputy Public Prosecutor Tang Shangjun said two months after Lim's marriage in 2011, his wife asked for a divorce.

Lim did not know then that she had been having an affair with one Nicholas Lee Ming Hong, 30, from April to June that year.

When asked, the 27-year-old nurse told Lim that he constantly had no time for her. Lim suspected her of seeing another man.

Lim's wife decided to end her affair in July, and tried to reconcile with him.

In August that year, Mr Lee began receiving harassing and threatening emails and phone calls, accusing him of being a cheat, and alleging that he had "stolen" another person's wife.

During one such call, the caller threatened to burn down Mr Lee's house and harm his family unless $50,000 was transferred to the caller.

Mr Lee told Lim's wife about the calls and emails. When she asked her husband, Lim vehemently denied the allegations. He retorted that if Mr Lee suspected that he was involved, he should make a police report.

In November that year, the flat where Mr Lee lived and shared with his elder brother, Mr Sean Lee, was padlocked and splashed with paint. Lim's wife again confronted her husband, who became upset because he felt that the allegations against him were untrue.

Worried that Mr Nicholas Lee would make a police report against him, Lim started accessing Focus to search for any reports that might have been lodged by Mr Lee against him. He used three of his colleagues' usernames and passwords to access the system.

The offences came to light after Mr Sean Lee, a 36-year-old teacher, made a police report on a case of unlicensed moneylending harassment at his flat.

A few days later, Mr Sean Lee found out that his name and identity card number were spray-painted on the gate of the school that he was teaching at. He suspected that a police officer was involved.

Police conducted an audit trail of the Focus search logs and discovered the instances of unauthorised access by Lim.

Mr Tang said there is no evidence linking Lim to the allegations of harassment at the Lees' flat, or the threatening phone calls or emails.

Sixty other similar charges against Lim were taken into consideration.

Lim's lawyer Nirmal Singh said his client and his wife have reconciled and are the parents of a newborn.

Lim could have been fined up to $5,000 and/or jailed for up to two years per charge.