Posing as an off-duty police officer, an odd-job worker told a couple he knew what they had done and that they could settle the matter the "non-government way" by giving him money, valuables or a "happy hour".

Kumaresan Raman, 28, had approached Mr Aravind Naidu, 19, outside a public toilet near Block 433, Ang Mo Kio Avenue 10, on March 29 last year while the teen was waiting for his girlfriend to use the toilet.

He claimed that he knew what Mr Aravind and his girlfriend, also 19, had done in there. Asked by the victim to explain what he meant, Kumaresan refused to answer.

Kumaresan said he was from a "government agency" but was off-duty and did not have his warrant card. He asked Mr Aravind to produce his identity card. He also told him to fetch his girlfriend from the toilet.

When she approached Kumaresan, he asked for her IC and told her that he knew what they were doing earlier.

Confused by his statement, she nonetheless gave him her IC as she believed that he was from a government agency.

He then spoke on the phone and pretended to verify their particulars.

Shortly before 12.15am, Kumaresan told them that they could settle the matter the "non-government way", which was to give him money, valuables or "happy hour", by which he meant sex with the girl.

He then threatened that if they did not choose the "non-government way", he would do things the "government way", which was to arrest them.

Frightened by his threat, they offered to give him $80, which he accepted.

On the way to the ATM to get more money, Mr Aravind, on the pretext of talking to his mother, called the police.

Kumaresan grew increasingly worried and kept looking around. The couple saw their chance to escape and ran away.

Sentencing Kumaresan to the minimum two years' jail and six strokes of the cane yesterday, District Judge Kenneth Yap said Kumaresan had not learnt his lesson.

He noted that Kumaresan had been placed on probation before in 2008 for posing as a public servant and cheating.

"Your behaviour is very disturbing, to say the least.

"What you have committed is not just a crime but also a crime of exploitation.

"You went to impersonate someone from a government agency. In doing so, you undermined the trust that is placed by normal citizens on public agents," said the judge, adding that Kumaresan was very lucky as the prosecution had asked for only the mandatory minimum when he deserved more.