A woman went to a holiday resort to celebrate the birthday of a friend she viewed as an "older brother" and ended up being molested by him.

The 25-year-old was groped while she was half-asleep and tipsy in a chalet room occupied by the man, his three siblings and their father.

On Wednesday, the 26-year-old man was sent to jail for one year and six months and given four strokes of the cane.

He pleaded guilty last month to outraging the woman's modesty. He cannot be named due to a gag order to protect the woman's identity.

The woman, who was the best friend of the man's sister, went to D'Resort @ Downtown East with her boyfriend at around 6.30pm on July 30, 2016.

After drinking alcohol, she went to a pre-booked room to rest.

While she was half-asleep, she felt someone touching her from behind. Assuming it was her boyfriend, she ignored it.

Deputy Public Prosecutor Gail Wong said the woman was shocked when she later saw through the open bathroom door the man sexually stimulating himself.

When he stepped out of the bathroom, she asked him what he had done to her. He replied "nothing" and left the room.

The woman then noticed that the other people in the room were the man's three younger siblings and their father. All four were asleep.

Later, she told police officers who were attending to an unrelated dispute nearby what had happened to her and they arrested the molester.

DPP Wong said: "The victim felt outraged and distressed by the accused's actions. She had viewed the accused like an older brother before the offence."

The DPP urged District Judge Ng Cheng Thiam to jail the man for one year and six months, with four strokes of the cane.

Stressing that the woman was a vulnerable victim, she added: "It was easier for the accused to take advantage of the victim's half-sleeping state and the victim would have been in less of a condition to resist any sexual assault."

For outraging the woman's modesty, the man could have been jailed for up to two years and fined or caned.