A volunteer police sergeant was jailed for a day and fined $1,000 on Tuesday, for abusing the use of his warrant card.

The court heard that Jeffrey Chua, who was a member of the Voluntary Special Constabulary, had approached 23-year old Chinese national Cai Yumei along Lorong 33 Geylang for sex in June last year.

He flashed his police warrant card at her in a bid to get her to reveal her identification documents, as he wanted to be certain she was above 18 years old. This even though he was not on duty then.

While holding on to her papers, Chua - who works as an auxiliary police officer with Certis Cisco - repeated a previous request for a discount in the price for sex with Ms Cai, to which she refused and demanded her papers back.

The pair were spotted by two other police officers, who after questioning them, referred the matter to the Corrupt Practices Investigation Bureau for further investigation.

Chua was charged under the Police Force Act for conducting himself in a manner prejudicial to the good order and discipline of the Singapore Police Force. The maximum sentence for such disciplinary offences is a $1,000 fine or six months imprisonment, or both.